He needs to supercharge turnout of his voting base in a few key states such as Wisconsin, Georgia, and Michigan. Vance is great for that.He picked someone that won't bring any new voters. That dude is hitting the same notes than Trump. That's a bad pick, but at this point maybe Trump doesn't even need that to win.
If Biden was in a stronger position, I could see him picking Vivek or even a wild card like Kennedy or Gabbard.