Indeed, what IS vintage?
In terms of the actual, physical guitar, it means that the instrument is over 15 or 20 years old. This does usually mean that the shops selling them generally add a few percent to the price, or is that me being cynical? :?
Soundwise, like indy said, it could mean a 70's Les Paul through an original 70's Marshall Plexi, or a 60's Strat through a 60's Twin or maybe a 50's Tele through a 50's Bassman, or some such. It could be that the pickups have lost some magnetism, giving a mellower, older style sound. I like Peter's explanation of old records, sounds that you really want to emulate as they were something special. :?
Pickup-wise, for a humbucker, it's the fabled Gibson "PAF", single coils could be back to that 60's Strat or 50's Tele, or even the jangly Rickenbacker based Beatles sound.
Dunno really, I reckon everyone has their own theory as to what "vintage" means, especially in relation to guitar's and how they sound. Some modern players want to recreate older sounds, as they think that they're more "organic", some players want to try and find the opposite for a new "fresh" sound. Who's to say. :?