Yeah, DEC Alpha was the first thing to come to my mind. I guess some might argue it wasn't a "PC" - if you don't include what used to be called "workstations". This is largely because PC meant "personal computer", and very few people could afford their own DEC Alpha - they were very pricey ($20k at some point in the 90's, I believe).
"Vintage" usually refers to actually old stuff, while "retro" refers to new stuff that looks/sounds/feels like old stuff. So GentleOS is a retro OS designed to run on vintage hardware.
(That distinction wasn't clear to me either, so I had to look it up - TIL).