Easy: Anything built before PC-XT. And some.
While I would agree that just about any computer made before the PC/XT is
'vintage', I think there were some machines made after that date that are
also vintage/interesting (Acorn Archimedes, some HP machines, some
PDP11s, CoCo3, possibly the PERQ T4 (I forget the exact date of the
PC/XT), etc). It would be a pity to miss out on those.
-tony