A number of people have tackled this question. Historian William G. Thomas III discusses it here.
Historian Dennis Frye speaks about it here:
Virginia is another Upper South state. Their secession is more complex than the secessions of the Lower South. Add to that the fact that the state didn’t publish official reasons for secession and our task takes some work to accomplish.
The Virginia General Assembly met on January 7, 1861, after South Carolina had passed an ordinance of secession. The first order of business was to read a message from Governor John Letcher. In that address, Letcher said, “For the present condition of public affairs, the non-slaveholding states are chargeable; and if the Union shall be destroyed, upon them will rest the solemn responsibility. Their systematic and persistent warfare upon the institution of domestic slavery, as it exists amongst us — their fierce and unqualified denunciation of it, and all who recognize or tolerate it, have done much to create the present state of exasperation existing between the two sections of the Union. Hatred to slavery and slaveholders is instilled into the minds of their children, as part and parcel of their education, throughout the infected district of New England. The institution is constantly assailed — through the press, in the pulpit, in public meetings, in private associations, in their legislative assemblies in their statues, on all occasions — as morally, socially and politically wrong. The slave owner is painted as the great criminal of the age, deserving death. Money is raised and has been expended in hiring desperate and depraved men, in arming and supporting them, in order that they may make raids into southern states, and excite the slaves to insurrection and murder. Arms peculiarly suited to the use of the slave, have been fabricated, and sent into the slave states, to be placed in the hands of this class of our population, after they have been stimulated to such a degree of madness as will qualify them for the commission of murder, arson, and every species of cruelty. The results of these teachings were seen in the Harpers Ferry raid.” The General Assembly passed a resolution against “coercion” and one saying that if the current crisis couldn’t be ended peacefully, Virginia would join with the slaveholding states.
“Resolved by the general assembly of Virginia, that the Union being formed by the assent of the sovereign states respectively, and being consistent only with freedom and the republican institutions guaranteed to each, cannot and ought not to be maintained by force.
“That the government of the Union has no power to declare or make war against any of the states which have been its constituent members.
“Resolved, that when any one or more of the states has determined or shall determine, under existing circumstances, to withdraw from the Union, we are unalterably opposed to any attempt on the part of the federal government to coerce the same into reunion or submission, and that we will resist the same by all the means in our power.” [Journal of the House of Delegates of the State of Virginia for the Extra Session, 1861, pp. 9-10]
“Resolved by the general assembly of Virginia, that if all efforts to reconcile the unhappy differences existing between the two sections of the country, should prove to be abortive, then, in the opinion of the general assembly, every consideration of honor and interest demand that Virginia shall unite her destiny with the slaveholding states of the south.” [Ibid., p. 67]
The Governor of Virginia, John Letcher, had said that he didn’t think Virginia should convene a secession convention, even though he believed there was a right to secede. “The legislature disagreed with Governor Letcher concerning the need for a convention, however, and after heated debate, particularly over provision that any decisions of the convention be submitted to the people for approval, a convention bill was adopted. The bill provided for an election on February 4, at which time voters would (1) choose delegates to the convention and (2) decide whether the convention had to refer its actions to the voters for their approval.” [Ralph A. Wooster, The Secession Conventions of the South, p. 141]
As Professor Robertson describes it, “February 4 was a cold, overcast Monday. Snow was falling over most of the state. Some 145,700 voters trudged to the polls. How they voted made it clear that party organizations and political platforms meant nothing. Voters gave more attention to individuals and personalities–in part because many people were themselves undecided on the issues and preferred entrusting settlement to their representatives. Further, Virginia citizens felt all along that the final say would be theirs to make in a popular referendum. The election of delegates showed no mandate for secession. Less than 20 percent of those elected were avowed secessionists. Unionists were delighted and momentarily optimistic. However, the term ‘unionist’ had an altogether different meaning in Virginia at the time. Richmond delegates Marmaduke Johnson and William McFarland were both outspoken conservatives. Yet in their respective campaigns, each declared that he was in favor of separation from the Union if the federal government did not guarantee protection of slavery everywhere. Moreover, the threat of the federal government’s using coercion became an overriding factor in the debates that followed.” [James I. Robertson Jr., “The Virginia State Convention of 1861,” in William C. Davis and James I. Robertson Jr., eds., Virginia at War 1861, pp. 3-4] We should at this point consider what they meant by “coercion.” ” ‘Coercion,’ to the moderates, meant the use of force for collecting the revenue, enforcing Federal laws, and retaining or repossessing the forts wherever the state as a political organization offered resistance. Whether these representatives believed in the right of secession or not, they believed that ‘coercion’ would bring about the subjugation of a state, and not only change the nature of the Union but destroy the South’s social and economic system. Furthermore, it would provoke a conflict in which Virginia would be forced to stand with the North or the South. Realizing this fact, some rabid secessionists asked the Southern Confederacy to precipitate matters in order to force the issue.” [Henry T. Shanks, The Secession Movement in Virginia, 1847-1861, p. 191] “Coercion,” then, was anything the government did outside standing aside and letting states do what they wanted to do, whether it was legal or not. Any attempt of the government to stand up for itself or to enforce the laws of the United States would be labeled as “coercion.”
There were actually two votes on secession. The first was held on April 4, with 45 of the 152 delegates voting for secession and 89 voting against. The second was held on April 17. At the second vote, 88 voted in favor and 55 voted against.
In analyzing these votes I divided the delegates into four groups:
Secessionists were those who voted to secede on April 4 [No one who voted to secede on April 4 changed their vote on April 17, but there were some who were absent on April 17. I counted them as secessionists]
Unionists were those who voted against secession on April 17.
Moderates were those who voted against secession on April 4 and changed their votes to vote for secession on April 17.
Unknowns were those who voted against secession on April 4 and were absent on April 17, those who were absent on April 4 and voted for secession on April 17, or those who were absent for both votes.
That gives us 45 secessionists, 55 unionists, 33 moderates, and 19 unknowns. Of the 19 unknowns, 7 voted against secession on April 4 and didn’t vote on April 17 [they could be unionists or they could be moderates], 10 did not vote on April 4 and voted in favor of secession on April 17 [They could be secessionists or they could be moderates], and 2 didn’t vote either time [they could be any of the three main categories].
Of the 88 who voted for secession on April 17, 45 [51.1%] voted for secession on April 4. Another 33 [37.5%] changed from being against secession to being for secession. Ten delegates [11.4%] who didn’t vote on April 4 voted for secession on April 17.
What were their motivations?
If we look at the percentage of the population in the areas they represented who were enslaved, we find an interesting distribution. This table looks at the distribution of delegates from areas with percentages of enslaved populations going horizontally and the groups vertically, with only two groups of unknowns considered, those who didn’t vote on April 4 and then voted yes on April 17 and those who voted No on April 4 and then didn’t vote on April 17:
This seems to indicate a significant proslavery component to delegates who voted for secession and a lack of slavery motivation for those who voted against secession.
Of the 88 delegates who voted for secession, 23 were in the Tidewater region, 32 in the Piedmont, 10 in the Valley north of the James River, 18 in the Southwest, and only 5 in the Northwest. Of the 55 delegates who voted against secession, 6 were in the Tidewater, 4 in the Piedmont, 17 in the Valley north of the James River, 3 in the Southwest, and 25 in the Northwest. [Henry T. Shanks, The Secession Movement in Virginia, 1847-1861, table on page 205]
In his analysis of the April 17 vote, Professor Henry T. Shanks wrote, “East of the Blue Ridge, the delegates voting in the negative were from Franklin and Henry counties which were on the edge of the tobacco belt in the southwestern Piedmont, Norfolk and Portsmouth in the southeast, Accomac on the Eastern Shore, Henrico in which Richmond was located, and Alexandria, Fairfax, and Loudoun which were near Washington city. The position of those from Franklin, Henry, and Henrico are not easily explained, for the neighboring counties supported secession. It is doubtful if the stand taken by these delegates represented the sentiment of their constituents. The other Union votes from east of the Blue Ridge are more easily explained. Norfolk County was rapidly developing her truck farming, and the market for her products was in Northern cities. Along with Norfolk, Portsmouth and Accomac, in case of war, would be open to an attack from the United States navy. People from this section, moreover, were already dreaming that a great city would be built in their midst out of the trade from the Middle West and Europe. Railroads and the James river and Kanawha Canal gave them grounds for their hope of trade with the West, and a steamship line recently organized offered prospects for trade with Europe. If Virginia withdrew from the Union the Western trade would be lost. Likewise, the counties around Washington were connected with the free states in trade and interests. But the thing that carried most weight in their decision was probably the fact that they would be the border counties and open to attack. There were also some farmers here from the North. The delegates from the southwest were almost as unanimous for secession as those from east of the Blue Ridge. The valleys there were fertile, slaves were numerous, the plantation system prevailed, and the Southern Methodists were strong. This section’s only railroad outlet was over the Virginia and Tennessee which ran from Bristol to Lynchburg and from there over the Southside Railroad to Richmond. The James River and Kanawha Canal also extended into this section as far as Botetourt, and many expected it to be completed to the Kanawha River, thereby giving them another connection with the eastern part of the state. There was little fear here of invasion, for its valleys extending northeast and southwest were protected by mountains on two sides and the distance from the border was too far to cause any great concern. In the northern and central parts of the Valley and in the northwestern part of the state, however, the story was the reverse. Their delegates voted almost solidly against secession. In the former section, railroads, churches, and trade ties connected the people with Baltimore, Washington, and the North. Some of the counties here were mountainous and without many slaves. They, too, were open to attack. Like the Valley, only more so, the northwest was bound to the free states by economic ties. Throughout the period of the Convention their representatives had reminded the east of this fact. Their products, grain, hay, cattle, hogs, oil, minerals, and wool, found their best markets in the North, and their railroad and river systems gave them their most satisfactory connection with the free states. Between northwestern and eastern Virginia there were few things in common.” [Henry T. Shanks, The Secession Movement in Virginia, 1847-1861, pp. 207-208] The eastern part of Virginia was where the majority of slaves were located.
Jeremiah Morton represented Orange and Greene counties. Adding the populations of the two counties together, 51% of their population was enslaved. Morton was a secessionist. On February 28 he gave a speech to the convention in which he said, “Men in every branch of the business of life do not know how to shape their contracts because of the agitation every four years of this never-dying question of African slavery–I say, I want to see this question put to rest, not where it will spring up to disturb my children and involve them in utter ruin twenty or thirty years hence; but I want to put it where it will never disturb my descendants–for if there is to be bloodshed, and this question cannot otherwise be settled, I would rather give the blood that runs in my veins, to preserve that which is in the veins of my helpless offspring.” [William H. Freehling and Craig M. Simpson, eds., Showdown in Virginia: The 1861 Convention and the Fate of the Union, p. 6]
George Wythe Randolph, another secessionists, represented the City of Richmond, which had 30.86% of its population enslaved. In his March 16 speech to the convention, he said, “The greatest of all wrongs, one which in my judgment would require separation from the North if they had never otherwise injured us, is the translation of anti-slaveryism to power, the change from passive sentiment to energetic action. While the anti-slavery sentiment was merely speculative we had no right to complain; but now that it has become an efficient agent in the government, it is no longer safe for a slave State to remain under that government. Take the history of abolitionized Governments and it is a history of abolitionized people. Look at England, France, Denmark, and at their magnificent Colonies; the pearls of the Antilles, sacrificed without remorse. Look at Russia. Abolition mounts the throne and serfdom disappears. What right have we to expect better things from our Government? Will the Constitution restrain it? Abolition will soon have the power to make that what it pleases, and as the gentleman from Augusta [Mr. Alexander H. H. Stuart], well said, in his excellent report on the Harpers Ferry raid, ‘The whole argument against the extension of slavery is soon by a very slight deflection, made to bear against the existence of slavery, and thus the anti-extension idea is merged in that of abolition. Accordingly we find, notwithstanding the denial by the Republican party of any purpose to interfere with slavery where it exists, that the tendency of its policy is to its extermination every where.’ ” [Ibid., p. 59]
Secessionist James P. Holcombe represented Albemarle County, which had 52.27% of its population enslaved. On March 20 he told the convention, “The institution of slavery is so indissolubly interwoven with the whole framework of society in a large portion of our State, and constitutes so immense an element of material wealth and political power to the whole Commonwealth, that its subversion through the operation of an unfriendly [federal government] policy … would, of necessity, dry up the very fountains of the public strength, change the whole frame of our civilization, and inflict a mortal wound upon our liberties. … I believe that this danger is impending; that it is of overshadowing magnitude; and that there is no rational hope of escaping from it, but in the prompt severance of the relations of this Commonwealth through the Federal Government, with the free States of the North.” [Ibid., p. 62]
Secessionist James Barbour represented Culpeper County, which had 55.33% of its population enslaved. He gave a speech from March 30 to April 1 in which he said, “The question is what we shall do to protect our slave interest in Virginia, and you ask Delaware to send somebody to advise with you as to what you shall do with your 500,000 slaves. … Delaware has 1,798 slaves, and her idea about taking care of the institution of slavery is to let Black Republican politicians attend to it. She has elected a Black Republican member of the next Congress, and her policy is a gradual extermination of slavery within her limits. … Well, what next? Maryland has 599,000 white people, and 87,000 slaves; that is, there are seven white men in Maryland to one slave. … Arkansas … has 300,000 white people, and 111,324 slaves. The whites there are three to one. Missouri, with a million of whites, has 114,000 slaves–the whites being nine to one. … Mr. President, will this Convention close its eyes to the astounding fact that when we get into that border States Conference, Virginia and North Carolina will own one half of all the slaves in the States represented there? And yet they will hold but two out of the eight votes. … Virginia with her five hundred thousand slaves would stand there with only as much power as little Delaware would have with her 1798 slaves. Is that the way in this great crisis that the wise men of Virginia propose to take care of the slave interests of the State?” [Ibid., pp. 110-111]
Middlesex and Matthews Counties, when combined, had 46.99% of their population enslaved. They were represented by the secessionist Robert Montague. In his April 1-2 speech he said, “The generation that now governs the North have grown up in the last 30 years, and [almost] every man … believes in their hearts, honestly before God, that slavery is a sin. Judge Story, Chancellor Kent, and Daniel Webster, in his great argument before the Supreme Court of the United States have taught them the fallacious doctrine that this is a Government of one people and … that one section must be in part responsible for the sins of the other section. … Judge Story’s book is a text-book in all their Colleges. Dr. Wayland, a great theologian, … has written a book upon Moral Philosophy; … his book is a text-book in all their Colleges; and … a whole chapter [is] put forth to show that slavery is a sin. Then, sir, there is Peter Parley, useful and instructive as his books are–yet … you will find pervading the whole works and anti-slavery feeling. … Look again at the way in which these Northern people have been educated. They have had anti-slavery catechisms, anti-slavery priests, anti-slavery lectures; they have anti-slavery everything. Their whole system is pervaded with anti-slavery feeling and anti-slavery sentiment. And yet Virginia statesmen get up here and talk about the overwhelming tide that is to destroy the anti-slavery feeling of the North, by paper guarantees attached to the Constitution. You may just as well attempt to dam up the tumbling waters of the Niagara, with your little finger, as to attempt any such thing.” [Ibid., p. 119]
Secessionist George Richardson, representing Hanover County with 55.06% of its population enslaved, spoke on April 3-4, saying, “To the Southern Confederacy alone can we look for a community of interest for strength, and for real sympathy in the maintenance of that most sensitive and vital of our domestic institutions, slavery.” [Ibid., p. 124]
It seems, therefore, that those in the Secessionist category were primarily motivated by the protection of slavery, and they provided 51% of the votes for secession on April 17. What about the other 49%?
As Professor Robertson showed in the quotation above, Marmaduke Johnson and William McFarland, both moderates who changed their vote from “No” to “Yes” on secession, made it clear that if the Federal government didn’t guarantee protection of slavery everywhere, they would vote for secession.
Thomas Flournoy of Halifax County, a county with 56.17% of its population enslaved, was another moderate who changed his vote to “Yes.” On March 30 he gave a speech in which he opposed secession, saying, “If the North … shall accept … [our] amendments to the Constitution … that vote … will … be the final and everlasting overthrow of this fanatical Black Republican party. … The very arguments we have been using in the South, … that slavery is right, morally, socially, and politically, will have to be used by the men of the North, … [when] urging upon their people the .. amendments … demanded by the people of Virginia. [Then] … the Northern people … will also … [conclude] that slavery is right.” [Ibid., pp. 94-95] In the same speech he said, “But, as I remarked, if it awakes them not and they refuse to listen to the voice of reason and the demands of justice, we … will then shake hands with and part from them, if possible, in peace. … Virginia would thus be placed in an impregnable position, where no finger of scorn, reproach or contempt could be pointed at her. … And when she shall go out, if go she must, having made this last noble struggle to save the country, the free States of the border will unite with the South. … The interest of all the American people will be concentrated in this great republic, while the fanatical States of New England will be left to themselves, to suffer the … dreadful recoil of fanaticism.” [Ibid., p. 100] Flournoy, then, also appears to be motivated by protection of slavery.
Protection of slavery in fact motivated many of the Conditional Unionists. “In reality, the Virginia Unionists, as well as other Southern conservatives, were definitely ‘conditional Unionists’ who felt strongly that Negro slavery was more important than the Union. By insisting that the triumphant Republicans must renounce their non-extension of slavery principle, the Southern conservatives abandoned their potential Northern allies more readily than the Northerners abandoned them. Although most slaveholders in Virginia recognized that slavery was a universal anachronism outside the South, they refused to acknowledge that slavery must also disappear from the South sooner or later. Instead of considering how the outmoded social and economic institution could be transformed into free institutions with a minimum of trouble, the slaveholders spent their energy in futile efforts to convince the North, and perhaps themselves, that slavery was the foundation of civilization and would endure forever. The manuscripts of Wyndham Robertson, a leader of the Virginia Unionists, are replete with evidence that this slaveholder could not conceive of life in Virginia without Negro slavery and that he chose disunion primarily because he believed the Republican demand for exclusion of slaves from the territories clearly imputed that Southern slaveholding was immoral.” [Dean A. Arnold, “The Ultimatum of Virginia Unionists: ‘Security for Slavery or Disunion,’ ” Journal of Negro History, Vol. XLVIII, No. 2, April, 1963, pp. 115-116] Like Richmond delegates Johnson and McFarland, “Robertson [who was not a member of the convention but was a leading moderate voice in the House of Delegates] opposed the secessionists only as long as he thought the Lincoln administration would concede constitutional amendments to protect slavery. If such concessions were not made he was in favor of secession.” [Ibid., p. 125]
Although not a single delegate made any antislavery comments in the convention, support for slavery was not the only reason moderates changed their votes. Let’s remember the concept of “coercion” mentioned above. Anything the Federal government did to enforce the law in the seceded states was to be labeled “coercion.” We can’t be sure how many moderates changed their vote solely because they opposed “coercion,” but we have to recognize it as a factor, though it’s clear that it was a minority factor in Virginia’s secession. But why was it a factor? Was it solely a states rights issue? Historians William Freehling and Craig Simpson analyzed the convention and found, “Except for northwestern Virginians, the state’s Unionists usually assumed that if war erupted between the Union and the Confederacy, Virginia’s would-be peacemakers must become rebel riflemen. War, as usual, would force those on the fence to decide only one question: whom they most wished to kill. Most Virginians preferred to slay insulting Yankee coercers rather than erring Southern brothers. And even if Lincoln could not threaten slavery in a peaceful Union, slaveholders’ defeat in a civil war might savage the institution. The issue of black slaves aside, most white Virginians cherished a crucial supposed state’s right: the right of the people of a state to withdraw their consent to be governed. If the federal government coercively sought to force citizens of a seceded state to be governed without their consent, the first principle of republicanism would be annihilated. Seceding citizens would confront the first principle of enslavement: coercion without consent.” [William W. Freehling and Craig M. Simpson, eds., Showdown in Virginia: The 1861 Convention and the Fate of the Union, pp. xiv-xv] So for most moderates there was a dual purpose–the protection of slavery and the states right principle of secession. But even that wasn’t enough.
Intimidation also played a role. “In mid-March, delegate Marshall M. Dent (Morgantown) wrote his local newspaper: ‘Your readers cannot imagine the state of things here from the reports of the Convention in the newspapers. Every means is used to intimidate the [unionist] members of this Convention. Meetings are held nightly. Bands are hired who parade the streets followed by a motley crew of free negroes, boys and mad cops, who go around to the different hotels calling upon the well known Secessionists for speeches … and every Union man is denounced as an abolitionist! The members from the Northwest are compelled to daily hear citizens of Richmond … point them out with the remark that ‘there is where the abolitionists sit.’ ‘ [James I. Robertson Jr., “The Virginia State Convention of 1861,” in William C. Davis and James I. Robertson Jr., eds., Virginia at War 1861, p. 13] As Freehling and Simpson write, “A so-called Spontaneous People’s Convention furthered secessionist convention delegates’ determination to act without delay. This hardly spontaneous assemblage met in another Richmond hall, except when its members spilled onto the streets to stage fiery demonstrations. The conclave of mainly younger Virginia hotheads had gathered to force older Virginia Unionists’ reluctant hands. In and out of the alternate assembly hall, revolutionaries boiled with threats to kidnap the (Unionist) governor, expel Unionist convention delegates from Richmond, seize federal military installations in Virginia, and provoke an extralegal revolution. This illegal movement threatened secessionists’ legitimacy. a nonofficeholders’ rebellion also threatened Virginia’s political class, the seasoned pros who had long governed from above with the voters’ approval. One of the most seasoned professionals at the convention–and among the most exasperated secessionists–now deployed rebellious extremists to bypass constitutional stop signs. On April 16 former governor Henry Wise organized a preemptive strike of extralegal militia units, seeking to seize the federal government’s Harpers Ferry Arsenal on the Potomac River and its Gosport Navy Yard at Norfolk. Wise worried that, otherwise, the federal government would remove the choice armaments in these installations, so useful to sustain a revolution. He also feared that without decisive action the indecisive Virginia convention would never, ‘for God’s sake, quit talking.’ On April 17, in a dramatic convention scene shortly after the narrow defeat of Robert Scott’s alternative to secession, Wise intimated that his recruits at this moment marched toward civil war against federal troops. … Immediately after Wise’s April 17 announcement, the convention’s approval of Preston’s secession ordinance drew some sting out of the question.” [William W. Freehling and Craig M. Simpson, eds., Showdown in Virginia: The 1861 Convention and the Fate of the Union, pp. xv-xvi]
And then, there were those who, like the moderate delegate Timothy Rives, representing Prince George County and Surry County, together having 51.65% of their population enslaved, who, when he changed his vote and voted for secession on April 17, didn’t identify slavery or coercion as the reason for voting, but instead identified the fact that revolution had broken the Union already.
So of the 33 moderates, we know for sure that three were concerned with slavery and one simply gave up and decided that since the revolution was there he would join it. Perhaps seven other moderates were like Johnson, McFarland, Flournoy, and Wyndham Robertson and acted to preserve slavery [based on the distribution of enslaved people in their populations in the table above]. That would leave approximately 22 moderates, 25% of those who voted for secession, who were either intimidated into voting for secession or voted for secession to sustain what they believed was a right to secession and to protect it from “coercion.” It’s not possible to estimate the distribution between intimidation and coercion as reasons for changing votes. Based on the percentages of their represented populations being enslaved [see the table above], we can estimate that eight of the ten delegates who didn’t vote on April 4 but voted for secession on April 17 did so to protect slavery. If true, that means approximately 71.5% of those who voted for secession on April 17 did so primarily to protect slavery. With anywhere from 51% to over 70% of the support for secession based on the desire to protect slavery, it’s no exaggeration to say that protection of slavery was the driving force behind the convention’s vote to secede, even if there were other factors involved.
There was still one event left in the saga, though. On May 23, Virginians went to the polls and voted on whether or not to ratify the convention’s decision. “So asked the Unionists’ John Baldwin on April 17, after Wise’s provocative announcement. The troops’ extralegal strike, despaired the Valley moderate, would preclude the May 23 voters from deciding whether secession was wise. Instead, true-blue Southerners would have to decide whether to repudiate their brave sons who had risked their lives against federal tyranny. The skewed results of Virginia’s May 23 referendum vindicated Baldwin’s fears. The voters’ 85 percent landslide for secession swelled beyond the convention’s 55 percent margin for Preston’s ordinance. The ballooning margin showed that Wise’s illegal violence had helped change the very basis of legal decision. In the Lower South, military ambushes had had a similar impact. In early January, secessionist governors had seized their states’ federal military installations before disunion had been approved. These presecession military strikes, in the name of military necessity, had sometimes turned close contests over secession into landslides for disunion. A resulting question, everywhere in the secessionist South, was whether elections distorted by such illegal violence, and replete with other voter intimidations, yielded legitimate tests of whether majorities had willingly withdrawn their states’ consent to be governed.” [William W. Freehling and Craig M. Simpson, eds., Showdown in Virginia: The 1861 Convention and the Fate of the Union, p. xvii] The vote, therefore, had been influenced by events from April 16 on, including Virginia’s joining the confederacy on April 17 and the confederacy’s decision to make Richmond its capital and to move confederate troops into Virginia, all before the ratification vote.
The primary motivating force behind Virginia’s secession was the protection of slavery, though intimidation, resignation to the fact that the Union was already broken, and the desire to defend a perceived right of state secession also played subsidiary roles.
You can follow this link to the Virginia Secession Convention Proceedings.