I think Vance is running a version of the same gambit that Tred Cruz tried (and failed) to pull off, except that Vance was cunning enough to flip-flop and become a Trump sycophant first, then weasel his way onto Trump's ticket, rather than running against Trump, losing and only then trying to get into his administration afterwards, which is the mistake that Cruz, Lindsey Graham, Ramaswamy, Tim Scott, Chris Christie, Marco Rubio and so many others made.And play the loyal deputy with a view to winning POTUS in 2028.
After they ran against Trump and lost, its too late to kiss up then, because he will always despise them for running against him. He only keeps them around to mock and humiliate them.
Vance is using Trump as a stalking horse, letting Trump collect up all the base and supporters for Vance to then inherit when Trump falls, either due to his mental health or criminal woes, or something else. Vance thinks he is going to be the heir to the MAGA throne