Vance cannot use the 25th amendment. He does not dare come out publicly against Dementia Don. The MAGAs would get their gallows back out and attack the capitol again. Vance will have to keep Trump on as a figurehead and maneuver around him.
It shouldn’t be too difficult, though: flatter him a lot, and offer to take care of boring details so that Trump can have more “executive time.” Whenever they need him to sign something, just tell him President Obama is desperate to keep him from signing it. That will work for as long as Trump can remember who President Obama is, which should last at least half-way through this term.
Trump is not long for this world. His Alzheimer’s is pretty far along now, and getting worse. If he lives to see the end of this term, he will be so far gone he won’t know whether he is president or not.
Doonesbury characters do age, but not as fast as we do.