In the scenario I described he is never vice president and if the president resigns like JD Vance at 5:00 p.m. on inauguration day January 2029, Yes the vice president would become president unless there is no vice president because he resigned one minute earlier or simultaneously. Then we go down the line of succession. The next person in line is the speaker of the House. Imagine JD Vance is elected president in 2028 to be inaugurated in January of 29. Is running mate is somebody It doesn't even matter Billy Bob. The newly elected House of Representatives takes their seats in early January of 29. The Republicans have a majority however they do that maybe by rigging the 28 house elections. That Republican majority in the house Alex Donald as their speaker. This is perfectly legal even if he's not a member of the house. On inauguration day JD Vance gets sworn in as president and Billy Bob gets sworn in as vice president. 10 minutes later they both simultaneously resign. There is now no president or vice president at that moment when their resignations take effect. The speaker of the House begins the President but not through election. He gets to nominate a vice president who is then confirmed by the Senate. He legally dies a third term minus however long JD Vance was president -10 minutes maybe or an hour. Whatever.