Interesting question and I've been pondering it since the last time someone asked it here. I know for some folks the time when brake cables started being routed along the handlebar is the cutoff point for vintage. For me, the classic age lasted a bit longer: when top tubes stopped being horizontal and started sloping back towards the seat tube. I think quill stems disappeared around this time.

As for classics, not all vintage bikes are classic but the Kestrel mentioned today is a classic in my book. It was years ahead of its time and despite its age, it still looks modern and wouldn't look terribly out of place at a triathlon among the Cervelos and Felts.