Second place rarely gets you the VP slot, the exception being George H.W. Bush in 1980. More frequently second place leads to first place in the next contested primary. Third place gets you nothing, except for Earl Warren who way back in 1952 got the U.S. Supreme Court Chief Justiceship.
For Mitt Romney the question is whether he finishes with a middling 7 or 8 million votes or can push that up to a more respectable 9 or 10 million votes. Also, with not quite half the vote in, it looks to be a close question whether total Republican turnout in 2012 will top 2008.
|2000||George W. Bush||12,034,676|
|1992||George H.W. Bush||9,199,463|
|1988||George H.W. Bush||8,253,512|
|1980||George H.W. Bush||3,070,033|
+ 2012 results are incomplete with more voting to come.
In 1952 and 1968 when fewer states held primaries, it was possible to win the nomination without the highest popular vote. But it's been 44 years since the last brokered convention.