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.
Yes, the VP position can remain vacant indefinitely, or at least until the end of a term when another VP can be elected through the normal process. The only thing that is lost is the tiebreaking vote. Well, and the fact that if the President has a colonoscopy and is put under sedation nobody is available to be acting President. I don’t think the House Speaker is able to take that job for just a few hours like the Vice President can.
Kamala Harris was actually Acting President for 85 minutes in 2021,
and probably at least once after that.https://www.cnn.com/2021/11/19/politics/kamala-harris-presidential-power/index.html
(nope, she did it just that once. https://en.m.wikipedia.org/wiki/Acting_President_of_the_United_States )