If you are a Republican, it's troublesome because Vance put that Yale education to good use trashing the shit out of Trump in both print and in interviews, explaining why he was unfit and untrustworthy, and all of those interviews and articles are going to be put to use in the run-up to November. You just know either Harris will bring it up in a debate (if Trump doesn't chicken out) or Trump will be asked about the specifics that Vance said.
Vance has also said plenty of shit that stirs the left up. Pence, for all of his faults, prior to 2016, was pretty under-the-radar. Vance, not so much.
I'm still surprised Trump picked him, because I was pretty sure Trump has an enemies list saved on his iPhone, and that Vance would be up there. It's also not like Trump was in danger of losing Ohio.
And as @HornOnTheBayou said, Vance has said shit out loud that you're not supposed to say until after you win and have power, which goes back to stirring the left up and bothering moderates.
Vance and Harris are unifying the left in a way that is not good for Trump.