The following was as clear as mud.
Office as stepping stone to the Presidency
In recent decades, the Vice Presidency has frequently been used to launch bids for the Presidency. The transition of the office to its modern stature occurred primarily as a result of Franklin Roosevelt's 1940 nomination, when he captured the ability to nominate his running mate instead of leaving the nomination to the convention. Prior to that, party bosses often used the Vice Presidential nomination as a consolation prize for the party's minority faction. A further factor potentially contributing to the rise in prestige of the office was the adoption of Presidential preference primaries in the early 20th century. By adopting primary voting, the field of candidates for Vice President was expanded by both the increased quantity and quality of presidential candidates successful in some primaries, yet who ultimately failed to capture the Presidential nomination at the convention.
Of the thirteen Presidential elections from 1956 to 2004, nine featured the incumbent President; the other four (1960, 1968, 1988, 2000) all featured the incumbent Vice President. Former Vice Presidents also ran, in 1984 (Walter Mondale), and in 1968 (Richard Nixon, against the incumbent Vice President Hubert Humphrey). The first Presidential election to include neither the incumbent President nor the incumbent Vice President on a major party ticket since 1952 came in 2008 when President George W. Bush had already served two terms and Vice President Cheney chose not to run. Richard Nixon is also the only non-sitting Vice President to be elected President, as well as the only person to be elected President and Vice President twice each.
http://en.wikipedia.org/wiki/Vice_President_of_the_United_States
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