And as I pointed out, the body of the Virginia legislature that remained loyal to the Union and which was recognized by the Congress as the legitimate legislature of Virginia did vote to partition the state. The letter of the law was followed if, perhaps, not the spirit.
FWIW Lincoln had his doubts on the legality of West Virginia's admission but the president has no constitutional role in admitting a state so there was nothing he could do about it.
My grandfather who died 10 years ago at age 100 said his grandmother talked about Union soldiers who were at the voting places and turned away men who were not going to vote secession from VA. I didn’t think that could really have happened but former governor Caperton says basically the same thing in a history of WV that he authored.
Oh, I'm sure they thought up some justification for doing what they wanted to do anyway. But that doesn't make any logical sense. The same body cannot be both the original state, and the portion that wants to break off.