Isn't the vice presidency worth about one bucket of warm spit? I mean, why are Vance and Trump acting like she gets to decide policy?

The VP is an extra vote in the Senate. That alone can be worth it's weight in gold.The VP is an extra vote in the Senate. That alone can be worth it's weight in gold.

Because it's the only vaguely-normie-friendly way they have to attack her.

They planned their entire election campaign around running against Biden and they're incapable of coming up with a new plan so they're just cramming Harris into where they said Biden before

Because they believe, probably accurately, that their followers will eat it up. I'm less sure what response it will get from non-cultists.