Ambition, grievance, and Peter Thiel's money. The question is how quickly Vance invokes the 25th, optionally with a White House doctor declaring that Trump's had a stroke, or arranging one, or a "mental breakdown".

Would they even need to do that? Trump will probably spend his time golfing and giving speeches while Vance does actual policy stuff. Trump shows up to the White House once a week or month to sign whatever is put in front of him and get his photo. Trump is just there to keep MAGA on board.