1. JD Vance independent of Trump will have the same policies.
2. JD Vance will have enough popularity for a serious 2028 run. He might fall out favour with Trump as Trump tries to mount a bid for a third term, Trumpism might just generally lose popularity if policies lead to bad outcomes.
3. Dems don't figure their shit out. They should be able to take back some control in mid-terms, and then start to push their own policies, or at least credibly show that the most extreme policies from the exec branch don't have teeth anymore.
And you're assuming that the 2028 election (and 2026 for that matter) will be business as usual elections, against all evidence to the contrary staring us directly in the face.
The federal government doesn't control elections in the US so they don't have much power here. Also, firing all the FBI agents is a bad first step to using them for interference. They have no clue how to be authoritarians; to do that, you need to be popular and have the security forces like you.
Parties in the US aren't real, though the brand value is good at keeping the right kind of people in each one. They can't actually kick anyone out and people in the federal government have no particular control over same party people in any state.
From my view, the two parties are quite a bit more real than the 3 branches of government. The people funding state and federal elections are the same.
If you mean campaign funding, that's mostly Actblue (upper middle class Dem donors). Republican donors are like, five people, yes - but voters have their own opinions and they definitely don't vote for whoever has the most money. Because the Democrats have more of it.
The playbook is obvious - if Trump loses, Vance refuses to certify the election results due to "fraud", Republican states will produce alternate lists of electors that vote for Trump, and he will claim that in reality he just won reelection with the biggest margin in history.
How do we know this isn't just crazy conspiracy theory? Because they already did attempt the same thing in 2020, and this time they had the chance to vet the VP candidate for this scenario.
Then we can have a color revolution. He's not doing the right steps to prevent that, because he's annoying the security forces instead of supporting them.
I just checked. US Constitution has amendment #22 that says: "No person shall be elected to the office of the President more than twice". Is there a workaround? I doubt he can change that amendment. None have been ratified in the US for a squillion years.