This isn't exactly vintage, but since there are lots of folks on this forum that buy oldish bikes, this is a good place to start.
Why do you think there is so little interest in older,steel(DB Chrome moly-probably japanese tange etc) late 80's mid 90's MTB frames,and bikes?
I just "won" a couple of early mid 90's Gary Fisher steel MTB frames.They are "pre trek" vintage(nothing wrong with Trek that I can see). I'm pretty sure they are typical Chrome Moly DB frames-maybe Tange, but probably Japanese tubing. No one else bid on the Wahoo and a Koo Koo Hoo something or other frames.They went for $35 and $20 with $30(total) shipping. Granted, this is $42 each, but that seems cheap for a decent frame (paint not so good but not dented).
Why no interest? I doubt that the rear dropouts are horizontal,so you couldn't do an easy SS adaptation. They do make very good all around street, trail,grass, median, bikes.You get a decent ride, not too heavy(about 8lbs for a frame fork )durable, they can take large tires,so you get a soft ride, if you want, they take cantilever or V-brakes so you can actually stop-not like older street bikes with the old caliper brakes.In general they are more versatile than the older road bike frames.They usually have bigger diameter tubes, so they aren't as elegant looking as older road bikes.They do have a slightly "chunky" look to them.
It looks like older MTBs are being ignored in the "steel is real" movement.Maybe it is the vertical dropouts.Older road bikes usually had hori. dropouts,so they make good SS starting points.