Vintage is a term used to try to squeeze more money out of a prospective buyer on the basis of age and exclusivity magically making something seem better than it actually is
In practical terms, I think there's a 30 year timeline where the price and desriability of instruments suddenly increases, certainly it looks like early 80's Fenders have come in from the cold, even late 70's and early 80's Japanese stuff are appreciating rapidly too.
Personally, I think there might be a link to the age of the buyer, where having disposable income and a large dose of nostalgia makes owning an instrument older than you are a cool thing.