Concise Oxford English Dictionary, Tenth Edition, Revised, 2002:
vintage: adj. 1 referring to vintage wine. 2. denoting something from the past of high quality
Many online dictionaries give a definition along the lines of: "having appeal; classic" or "representing what is most characteristic of something".
As far as I can tell from internet searches, the only adjectival use of "vintage" that relates to specific dates are those for Cars and Clothing (and it seems that clothing experts disagree on whether anything newer than 25 years should be included as vintage).
So, for anything that's not a car or a piece of clothing, it's ok to call it vintage if it is thought that it 'has appeal' or is 'characteristic of its type'. And most of the Chinese weights certainly are characteristic of their type.
