Vintage, unless it's changed and I don't think it has, means, in the UK, built before the end of 1930. Full stop. No exceptions.
Classic means anything you like, including or excluding appalling bits of stuff that didn't work when new, don't work now and are best forgotten apart from for sentimental reasons. 'Classic Car' magazine is full of these bits of rusty worthless tin, with poor brakes, no steering, lousy engines . . . We in the bike world have not had to suffer quite the same indignity of seeing rubbish elevated to unmerited heights, although it is happening.
Veteran (in the UK) means anything built before I've forgotten when - but very darn early (1912?).
PVT - 'post-vintage thoroughbred' - means (in the UK) things of a certain persuasion which were made immediately after the end of the vintage era (up to WW2 I think), but regarded as worthwhile and in keeping with the pre-1930 stuff. They are eligible for VSCC events in certain circumstances I think. Don't know quite what qualifies.
I think the VSCC in the UK could certainly enlighten anyone that's bothered, as could the VCC (Veteran Car Club). The VMCC should know too! Not sure how other Historic organisations view things.
The 25 year convention is convenient, but also a millstone. Who wants to think of a Honda Superdream as a classic? A 3 1/2 Morini, yes, or a Ducati, a Guzzi LeMans or even some of the ground-breaking 4s from Japan . . . but not every common-or-garden bit of bent tin that wasn't much good then and shouldn't have its status elevated just because it's rare (because most of them broke) or getting old.
Not that I care THAT much!!