Here's the thing with Vance: Ambition. Boundless ambition. He wants desperately to be President. That meant sucking up to Trump to get his endorsement and to become a Senator.
Notice how clever our young hillbilly is. He got the VP nomination. He goes on the debate stage and says none of the outrageous things that endeared him to Trump to begin with.
If Trump wins, he's term-limited, and JD is the presumptive 2028 nominee.
If Trump loses, JD is the presumed front runner because he's a smarter, less emotional, more focused version of Trump.
And if JD becomes President either sooner (Trump is, umm, almost 80) or later (2028), what makes the Trumpies think he won't switch back to that Hillbilly Elegy guy running toward the center? As I said, he's smart. I think Democrats fear JD for that reason: he'll do whatever he needs to do to win, even if it means running the old Bush 41 "kinder and gentler" campaign at the expense of his mentor.