The real problem for Trump is that Vance is an entirely new weak spot. Everyone knows Trump at this point, he is actually quite difficult to attack because most people already have a pretty locked in opinion about him. It’s very difficult to change votes with a campaign focusrd solely on Trump. Vance is a lot more unknown and thus Democrats actually have a really good chance to shape how the public sees him. It helps massively that Vance is a genuinely unusual character. If the election ends up being as tight as is currently predicted, a bad VP pick could be very costly for Trump.