I think that while generalisable quantitative parameters such as 30 years can be used to define 'vintage', among musicians it more often describes a golden age, perhaps more like a wine. For example, I might be wrong but I see the word used a lot to describe 1070's Fenders/MMs/Rics - at least one of which had a golden decade in the 70's (I'll leave that up to you reader) - but less to describe, for example Carlsbro' or H&H amps from the same period, neither of which IME had a golden 70's. Probably a very subjective view