You know you might have a vintage trailer on your hands when:
1) You decide that it's worth more than it cost new.
2) The AS is older than you are.
3) The torsion axles have been replaced more than once.
4) It doesn't have torsion axles and never did.
5) All the original appliances have been replaced.
6) Replacements for the original appliances are no longer available.
7) Before Beatrice
8) You buy more mantles than light bulbs
9) They quit making the size of tire it had originally
Beauty, classic, and vintage are all in the eye of the beholder, and not everything becomes classic.
1957 Chevrolets were widely considered to be classics 20 years after they were manufactured, and have always been highly collectible.
1975 AMC Pacers will never be widely though to be classics no matter how old they get. I'm not picking on AMC. Ford Pintos and Chevrolet Vegas from the same era will never really be classics, either.
The best straight answer I can give is that a "vintage" trailer, like the vintage wines from which the term originated, has unique qualities that set it apart from current production.