So how do you deal with the fact that having a vanguard replaces one elite with another and you just end up hoping for...

If you know your theory, you don't need to trust anyone since you're able to tell the genuine vanguard from the opportunist.
If you don't, well, why would need your trust?

So were the bolsheviks a genuine vanguard?

Before Stalin? Sure.

B-b-b-because the s-s-state just muh-muh-muh-magically whithers revisionist liberal :(

Only, this is a baseless assertion

The state being the instrument of one class's oppression onto the others, how could it not wither away when there are no more classes?

Just as Lenin did?

Because historically it has not

No. Russia was full of worker-run soviets and the provisional government was chock-full of leftists. Lenin was just a crybaby that wanted to become a dictator.

So in other words, the genuine vanguard eventually regressed into another elite after the original leader died.