Since FDR passed away in 1945, we’ve had twelve presidents. Five of them (Truman, LBJ, Nixon, Ford, and Poppy Bush) were vice presidents before they took over the helm in the Oval Office. It’s true that three of the five took office due to death or resignation, but death and resignation (as well as impeachment and conviction) are ever-present possibilities. It’s clear that vice presidents are more likely to become president than people holding any other position. The fact that Joe Biden is currently the poll-leader among the Democratic candidates just reinforces this point.
Of course, there are no guarantees. Dan Quayle could never get any traction for a presidential bid. And just because you win the nomination, as Hubert Humphrey, Walter Mondale, and Al Gore did, doesn’t mean that you’ll ever be president.