Totally would have happened w/o Virginia. It would have been a vastly different war. I personally think it would have been over earlier, and there may have been a much different end to slavery. On the other hand, Virginia would still have been a hotbed of secessionist activity, so there is a real possibility of a large number of Virginians signing up for the Confederate Army (like what happened in Kentucky). So the manpower difference might not have been too huge from OTL.
There is also the leadership difference to consider. Robert E. Lee, Stuart, Jackson, etc may very well have remained in the US Army, so the flavor of the early battles in the east would have been interesting.
TBH I can't think of a particular POD that would keep Virginia in the Union, at least not before like 1850 maybe? The pro-Union areas seceded from VA to form West Virginia, and in OTL were not enough to keep Virginia from initially seceding. I can see maybe Tennessee staying (or being neutral like Kentucky tried to do) or even Texas staying before Virginia did.