When was the last time we had a sitting President not running for re-election and his Vice President was not a potential candidate to replace him? Did Truman's VP Barkley try to run in 1952?
You have to remember that candidates didn't run in the same sense as they do today. Alban Barkley was in his mid-70's and in poor health in 1952 but Truman would have had him nominated in order to block Kefauver if that was the only way. In the end Stevenson got off the fence and got the nomination. Barkley died in 1956.