Are "vintage" DAC's worthwhile, or is this a tech that does not age well

All competently-designed DACs will sound alike. This applies to ones from 20+ years ago to the present. There is NO ever-upward increase in sound quality. The proliferation of ever more expensive DACs is a marketing ploy based upon confirmation bias to get the gullible to spend more!

Clever marketing psychology drives the market for new DACs. In controlled A/B tests nobody can distinguish a $100 DAC from a $1000+ one. The loudest complaints against this will of course be from those with the most money invested. 

Back in the 80’s the first test done was comparing the Theta DAC (designed by Mike Moffat) against the Sony Discman. Electronics used were by Boulder and speakers were Quad 63’s. After many trials the final scores were no better than 50/50.

I would not buy a vintage dac simply because of the service issues.  Some companies just have lousy or hyper-expensive service costs, not to mention the possibility of obsolete parts.  I'm not saying the newer dacs necessarily sound better than the best old ones, but they are certainly a safer bet.  As to the new ones, there are so many dac threads here, I'm not even going to get into which ones you should consider.