Hahaa....
The good one. Just my idea...
Lets say...by manufacturer --
There are "new" and currently "in production" synths, then there are "supported" instruments, but also "discontinued" products, and "no longer supported" products. Still, if it takes just a 10 years to reach to this point it does not certainly mean vintage by that term. So, it must be even older.
But... what pops into my head is that all these "vintage synths" have in common --
they all have atleast one detail which is hard to obtain *now* (or there are much better replacements instead of that!), but that detail was widely (by truckloads) available before and the use of obscure tricks to get over the technology limitations in that time -- whatever if it is huge size capacitors and resistors or small capacity 16..1024kBit SRAM chips or some obscure transistors for which are now more than 10 better alternatives and it is there, but not in the shop by now!
Also by technology means they have some tricks used which was done becouse of the technology of that time.
(Floppy disc drivers and floppy discs (hey, where from You get those now?!), 32kHz sampling rates, channel multiplexing). All becouse that was the time and the technology by then.
So, perhaps this is enough which classifies the synthesizer as a vintage?
In some cases there are better alternatives and so all can be substituted with modern details, and capacitors are much smaller for the same capacity, but what I mean that You cant get the same details directly from shop.
Well, the corporations proprietary chips are always outdated by next season and hard to find, unless You work there.
When I dig into some synth and look up its datasheet and I see "not recommended for new designs" or even "no longer in production", then it must be a vintage synthesizer.
Still, here are listed the synths which does not follow that criteria and are just couple years old and all parts available.
Whereas the Prophet 6 is not a vintage. It has too much modern technology inside, along with digital effects onboard, all of which are available. But it would be automatically vintage by my criteria if it would have atleast
a) common average floppy disc drive which was produced 30 years ago which was widely available by then,
b) some old SRAM memory chips, instead of flash memory
c) electrolytic capacitors which are 12V 1000uF, but with a size of 20x90 mm.
or 4000uF 25V capacitor which is in a size of 0.25 litre bottle. (this was common by 1990).
Take your pick.
BY technology I would say that having a solderable battery onboard would make instrument vintage too perhaps (or if they have battery at all! Since that trick was used by that time on older synthesizers but vanished with progress of the flash memory?) Still, all those batteries are widely available, which maybe is, maybe not criteria when looking this way...
Also carbon pots do not make the synthesizer real vintage as they are widely available by now, and just that the company provides small limited supply, does not count (Behringer has always provided only limited amount of spares).
Of course, when the term started being used in the mid 1970s, guitar collectors were looking at 1950s and 1960s electric guitars made before the major mfrs all got swallowed up by conglomerates, so some "vintage" guitars were only 10-15 years old at the time.