I feel dirty saying good things about Sarah Palin, but she was brought on to fire up the base and did just that. Vance has managed to alienate the base. I don’t think the incels even like him at this point.
Sarah Palin was like a dog that peed on themselves because they're an idiot. Vance is like a dog that bites their owner because they're an idiot. Sure, Palin was dumb, but Vance is a real threat to all Americans.
But since then, he has faced a rash of bad headlines about his past comments calling Harris a "childless cat lady," his past support for a national abortion ban and even a baseless internet rumor about having sex with a couch.
It may be baseless, but it's quite plausible because he is just so...uh...the way he is. Trump, Vance, and RFK are all kinda weird. Hugging and kissing the flag, playing with dead things, etc etc.
Considering that Vance's biggest accomplishment so far is writing a best-selling book, in which he cosplays as a poor person while trashing rural Americans, this about checks out.
Palin alienated anyone vaguely in the middle, but on the other hand she had at least some particular support that she brought distinct from McCain (basically Tea Party folks that were not particularly excited by establishment republican nor McCain).
Vance doesn't have any particular support, he only gets indirect support for people who are "republican no matter what" or "MAGA all the way", but those people were already covered by Trump. Even in his own state his margin of victory was much slimmer than the other republicans that swept Ohio in 2022, and Ohio's not even a state considered to be a risk to Trump's campaign.
He was just a tremendously bad choice with no strategic benefit. The VP pick should somehow provide some coverage for demographics not hit by the presidential candidate, and Trump instead went with someone with very limited popularity that is a strict subset of people already supporting Trump.