Well, I really don’t have to prove it. But I haven’t ever heard anyone saying “no” to a vice presidency of the United States, a position that’s just a heartbeat away from the top job.
Well, yes you do. And I will bet there have been a few that said no.
It happens all the time, for a variety of reasons. A potential VP candidate doesn't want to be associated with a losing campaign, desires to stay in their home state, wants to go into the private sector, etc. Generally the person will make it clear in advance that they have no interest in the office.