Perhaps, just that he's male, and white, and southern, and said that lonely cat lady thing. Makes me wonder if Trump's metrics might not have been /strategically/ better with a woman, or a minority, or someone with more appeal to the youth base, especially ladies.
Aside from the fact that Vance started out a Never-Trumper, his appeal is more to the Midwest and deep south, the two places that Trump needs the LEAST amount of help with.
But to his credit, I see Vance as a very CONSTITUTIONAL candidate--- it seems that while republicans tend to select people based on institutional qualities of what is best for or offers the most for the nation, democrats seem to pick people based on demographic or strategic advantage, like maybe picking Shapiro to secure the Pa vote.
The good news is that so far, I see the Walz pick as the first big strategic mistake that will hurt Harris.