I think the requirement is an interest in things that are no longer made, or at least things that resemble things that are no longer made, plus anything else that interests us.
I like this definition...it's what the owner of the bike thinks. I personally find the C and R list too rigid, but it's not my list.
My personal definition is that vintage is 25 years and older...
Classic is something that people either lusted after 25 years ago or will list after 25 years after it came out. This leads to subjective opinions...to me my Merlin is classic because I strongly suspect they will be lusted after.
I personally think that C and V is about the best bikes of their periods. To me, a CF bike that is the best in its class and that people will likely lust after in 2050 is more classic than a 1975 gas piper that had millions in the production line. The 1975 gas piper might be vintage, but it's not classic. I am more interested in classic than vintage. A garbage bike doesn't suddenly become special to me after 25 years...it's just an old garbage bike.