Honestly I don't think Vance as VP was a mistake, I actually think Waltz as VP was a mistake by the Harris campaign considering they were running some Republican lite campaign, there's many VP candidates better for such a campaign. While Waltz had the initial momentum on his side with the jokes and Vance's awkwardness, this quickly fell to the wayside after the VP debate. In that debate, Vance beat Waltz convincingly, which started Harris' downturn in the polls. The first domino to fall, so to speak. Waltz fumbled the bag hard during that debate, which began the shift of momentum from Harris to Trump.