Lots of questions. Here are my views:
Firstly, we need to define 'vintage'. I'm going to take a punt at anything up to the mid-70s. Obviously that's up for debate.
Secondly, I'm thinking of Fender Ps, purely because those are what I know something about.
Is a refinished body worth more than a new body? Well, a vintage bass in original condition except for a refin will fetch more than a new MIA, so, in that respect, yes. Whether it's actually better is another debate.
I have refinished a bass, and there was no discernable difference in tone.
In general, people do NOT refinish a bass in order to sell it. The market has shown that a refin will significantly DECREASE the value of a vintage bass. The refinish is usually something that has been done much earlier in its lifetime. It could have been done simply because at the time it was worth less, and a refin didn't change the value that much. Or maybe they just thought it was a keeper.
So, no, no one in their right mind would refinish a vintage bass to sell it, and no collector in their right mind would want a refinished bass over one in original (albiet worn) condition.
Mine is a '78, so not vintage by my standards. It started out life olympic white but by the time I had bought it in 2001, it had been stripped to natural bare wood. After about eleven years, I fancied a change and painted it orange! I never intend to sell it, but if I did, I'm guessing that it's as valuable now as it was stripped.