Like lots of other words, Vintage is over used (as is Mint). The word was usually accompanied by a date, as in, " this fine wine is Vintage 1936 ". In the motoring world there are specific dates, with Vintage coming between 1918 and 1930 after which it became it Post Vintage.
I think with everything else it's become a good descriptive word for sellers of old stuff to add some respectability.
Mel.