Here's an odd data point: Guitar Center seems to define vintage based on a combination of age and reliability of pricing data. I recently sold them a long-ago discontinued effect pedal... nothing special... about 25 years old... valued under $100. I presented it to them as something to buy for their "used" inventory. The local GC salesman said it would actually be classified as part of their "vintage" inventory, and he even had to contact GC headquarters for authorization on a purchase price, because of its relative rarity in their system.
This classification was somewhat unrelated to the age of the pedal in the sense that, had I brought in a 25 year old common effect pedal, especially one still in production (e.g., Boss overdrive), they would have bought it as "used," and the local guy would've had authority to do the transaction without HQ.