If it's after the election but before the electors vote in December, the electors are free agents. The party would probably pick someone and tell him to vote for them, though.
If it's after the electors vote, the vice-president-elect becomes president-elect.
At least, that's my understanding.
Horace Greeley died in 1872. Most electors voted for the VP Candidate B. Gratz Brown, yes.
Tom Eagleton resigned in 1972 (just a coincidence?) R. Sargent Shriver was approved by the Democrat National Committee (not the convention).