Nixon's original VP, Spiro Agnew, resigned and was replaced by Gerald Ford, but that was a result of scandal, not a re-election decision.
Franklin D. Roosevelt, in his four terms, went through three VP's. He had John Garner 1933-41, after which Garner retired from politics, and was replaced by Henry A. Wallace as his running mate for his third term (1941-45). Roosevelt opted not to run with Wallace for his fourth term, choosing instead Harry S. Truman. Shortly after Roosevelt won his 4th term, he died of a cerebral hemmoraghe, and Truman became President.
There are a few more incidents of VP's being replaced mid-stream if you go back before the 1900's, but that's not quite as relevant to our modern situation.
As for incidents where a VP was replaced, and the ticket did not go on to win...
Eisenhower v. Adlai Stevenson (1952). Eisenhower keeps Nixon for both terms (and wins both elections). Stevenson runs first with John Sparkman, then again in 1956 with Estes Kefauver, and lost again.
Carter kept Mondale both times he ran (losing in the second term). Reagan kept Bush Sr. for his two terms, and Clinton kept Mr. Roboto for both of his.
I think that's all correct.
Edited to add; George H.W. Bush kept Quayle as his mate for both of his runs, losing the second to Clinton.
Edited by: The Partisan at: 3/3/04 3:08 pm