Vecna is already a god, he doesn’t need to do that again. Been there, done that, bought the tee shirt. And why should the players care about one more god when there are already dozens? So yes, it absolutely matters.
And because he has been a god and moved beyond he can play twelve dimensional chess with reality. His plans seem vague because they are beyond human comprehension.
If you are having problems with this adventure it’s not because your stakes are too high, it’s because they are not high enough.