I think he will step back and bide his time. Several conservative commentators have described 28 as a "poison challice" of sorts for the Republican nominee. It is basically Vance's nomination if he wants it, but he faces an uphill battle after the midterm wipeout and the growing obviousness of how lame a duck Trump is at this point. Vance is going to struggle to distance himself from most of the negatives of this current administration.

If someone like Rubio gets a VP nod, on that losing ticket, it doesn't necessarily hurt a potential run later when the conditions could change again entirely. Whereas if he engages in a primary fight vs. Vance, DeSantis, etc. over who runs at the top of the ticket in 28, even if he somehow gets the nomination, he is kinda just setting himself up to be the fall guy (I'm assuming of course that 28 continues the trend of backlash against the current administration).