Like any subjective term in the English language, the definition of vintage when it comes to computers can be debated at length. In the end, it tends to be a consensus opinion.
At a site I frequent, the vintage computer forums (http://www.vintage-computer.com/vcforum/
), they take the view that it's anything from a PC-compatible 486 downwards (about 1996 and earlier..and whatever the Mac equivalent is at that time). That's generally a definition I agree with and one that I've used for this site. Anything more modern than (Pentiums, Celerons etc.) are more obselete than vintage IMO.