I think that when you stop equating 'vintage' with 'valuable' it's easier to make sense of the term. With musical instruments, 25-30 years old or more seems to be the concensus where the term 'vintage' applies. Whether or not a vintage instrument is valuable or not involves so many other factors!
For instance, I have a 60's Decca bass (re-branded Teisco) that is by all descriptors a vintage instrument. However, it has no appreciable value advantage from that fact and is only worth 100-150 bucks.
As long as you don't let yourself get sucked in purely by the term 'vintage', confusing it with 'valuable', the terminology is easier understood and less polarizing.