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

You did not say how much you were planning to spend. I would not invest in a old dac though 
but a nice R2R dac like a Holo springs May dac,
Denafrips R2R dacs ,even black ice  audio burr brown tube dac ,Topping 9
many options depending on budget to spend ,many prices and different flavors depending on your audio systems sonic signature.

I have found that at least on the few dacs that I have tried that you can put a very good modern USB to SPDIF converter in front of it and get excellent up to date USB performance.  Also,  I purchased a SMSL M500 Dac and was pleasantly surprised at its performance.  It definitely out performs the other in expensive DACs that I've had in my system. and believe me, I wasn't delusional.  The differences were very audible.  I have now owned 11 different inexpensive DACs,  many were similar, but some varied wildly.  

I am listening to a Dac built in 1995 and it sounds sublime. There's a lot more to Dacs than the latest digital technology.

FWIW I tried a few of the top DACs recommended by Audio Science Review from Topping and others.  Thought they were clean and measured well, to my ears, they were completely uninteresting and added nothing. To many that’s exactly what they want and I can’t fault that. However, I ended up with a MHDT Orchid using a “vintage” NOS DAC chip. It captured more of the “analog” and “vinyl-like” smoothness and life that vinyl has and added something to the music which I enjoy and which the Topping DACs did not possess. An engineer or a measurements fanatic might cry heresy but hey, it’s all about what sounds good to you. 

Agreeing with above. There’s only so much you can do with A/D conversion of 0’s and 1’s, as proven from being in studio sessions hands-on.