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.