By the time the referendum was held the Virginia militia was already mobilized and had seized and looted the federal arsenal at Harper's Ferry and the shipyards at Norfolk.
The governor had already expressed his intention to joint the Confederacy and agreements had already been reached between Virginia and the CSA well before the referendum.
There was even agreement to move the Confederate capital to Richmond by that time, so no one was in doubt about what a "yes" vote to secession in the referendum would mean.
Not to act after the referendum -- when it was clear how things were headed -- would have meant losing more ground to subversives elsewhere in the country.
More here.
Even after Fort Sumter, the Virginia legislature had voted to remain in the Union.
If you mean that the Virginia convention had voted to remain in the union before the surrender of Fort Sumter, that's true. I can't find what you said, though -- you'd have to prove it.
The Civil War only lasted 4 years. It has gone on at least 12 years here at FR. :)
To his credit, Lincoln waited until the people had had their referendum before initiating his invasion of Virginia. But the bottom line here is that Virginia was not allowed to peacefully secede against the assurance they had been given in 1788. Hostilities in Virginia were initiated by the invading army of the North.