I think all of your Dem friends were right, but it sounds to me like they were basing their comments on an incorrect understanding of the cause/effect relationship with Kerry. Kerry didn't lose because he was a U.S. Senator -- he was a U.S. Senator because he's a loser in every sense of the word when it comes to exhibiting the qualities of leadership that voters like to see in presidential candidates.
The problem with a U.S. Senator is that he is caught between two worlds. He spends most of his time in the national spotlight in Washington, but still represents his home state. Therefore, he often takes positions as a Senator for the benefit of his home state that don't go over well with voters in other parts of the country. As a result, he spends a lot of time on the campaign trail explaining his prior track record, twisting himself into knots explaining away his votes, and sometimes even blatantly talking out of both sides of his mouth. (Hmmm -- does this sound like John Kerry, or what?!)
A governor, on the other hand, almost never has to apologize for the positions he took in his executive capacity. Even his most controversial actions rarely affect the nation as a whole, and most voters are smart enough to understand that the governor of State X only serves the voters of State X. The fact that most governors are considered "outsiders" in Washington reinforces this image even further.
Nothing you said about a senator applies any less to a senator who also happened to be VP.