They would have to do that before the power in the Congress changes. An appointed VP has to be voted on by Congress-both houses. I doubt Hillary gets through a GOP controlled Congress.
See my post #43. Yes, the 25th amendment governs. Newsom or some other Dem other than Hillary. Whoever replaces Biden will probably be faced with a Rep-controlled House, which will block the Dem agenda. And then there are the investigations that will damage the Dems before 2024.