Why JD Vance.

My current guess?

JD was chosen specifically because Trump is feeling supremely confident he'll win.

Instead of picking a VP specifically to help win an election, which is generally the consideration, I think Trump was asking who could carry the torch for Trumpism four years from now.

Kamala was picked because she was a black woman despite the fact that it was obvious succession could matter with Biden's age. Pence was chosen to help unify a fractured Republican party. Kain was chosen to be a non story, while Palin was meant to shake things up. Biden was chosen to be a white male with previous missteps on racial issues.

Point is, all these choices were political calculations with the goal of winning the election in mind. I think by contrast, picking JD was an attempt to have his philosophy (to the extent he has one) outlive his Presidency, because he has pretty good reason to think he'll win regardless.