The diffence between Vietnam and Ukraine, is that Vietnam is 1000's of miles from America, Ukraine on the other hand is on Russia's border and is considered and existential threat to Russia it self.
Russia cannot just withdraw unilaterally as this would mean NATO forces could move right up to the Ukraine /Russia border.
But the Ukraine are not in nato
And highly unlikely to be granted member ship in the next 20 (?) odd years ???
Russia now has potentially two more borders with nato
Finland and Sweden ?
Would have to agree that the Ukraine will lose territory in the east
The Russians will not give it up