Classic implies quality. Vintage implies age

Classic
There are two generally accepted dictionary definitions for the adjective "classic" (below). I think we're talking the first one here. Since judgement is implied, I interpret this to mean "it's whatever you want it to be".

adjective: classic
1.
judged over a period of time to be of the highest quality and outstanding of its kind.
"a classic novel"

2.
remarkably and instructively typical.
"Hamlet is the classic example of a tragedy"


Vintage
I think we're talking a combo of 9, 10, and 11 here. Age is the key thing. Of course, one person's vintage bike is another person's used clunker. Most of us here focus on pre-mid-80s as the vintage era of interest. However, there are no hard and fast rules.

adjective: vintage

7. of or relating to wines or winemaking.

8. being of a specified vintage: Vintage wines are usually more expensive than nonvintage wines.


9. representing the high quality of a past time: vintage cars; vintage movies.


10. old-fashioned or obsolete: vintage jokes.


11. being the best of its kind: They praised the play as vintage O'Neill.