It has never happened before, so we don’t really know. What I think would happen is that Vance would just get immediately sworn in as the President on Jan 20 if God exercises His Ultimate Veto on Trump’s second term.

But the Presidential Succession Act only covers vacancies for the Presidency. Once the office of the President is filled again, nothing else happens. It’s not like everyone else “moves up” a slot. Mike Johnson would probably see the VP position as a demotion honestly.

The Office of the VP would remain vacant until President Vance nominates a candidate, and then it must be approved by both houses of Congress in order for the position to be filled.