Please list all the candidates that lost because of their VP choices. I quickly looked back in my mind (old and failing sometimes), and I don't recall any. Of course, I only only have studied the campaigns since WWII.
In US politics, the VP choice rarely makes any difference. He or she is usually an attack dog in the campaign. In this election, one might worry about Biden's age, so the choice will be more important than most.
In the end, the election is about Trump: "yes" or "no".
The election is also about turnout.
We often talk about the middle that needs to be convinced. For now, I just don't see this perspective as being applicable. The Democrats need to get their voters out. Trump needs to get his voters out. It is really that simple.