Whilst that’s definitely old in computer terms, even “retro”, is it old enough to be “vintage”?

Personally I’d have said it isn’t. But these terms are subjective.

"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).