Vintage is just old. I'd say 25 years old or older. Classic IMO can be a bit younger, but has 'classic' attributes. For example the 2011 Masi Speciale Strada-new bike but built with classic lines and attributes.
As far as a true classic, I feel it had to have at least a following at one point. I would say any nicer bike without the new fangled brake levers from outer space that shift the derailleurs, odd blade shaped spokes, no plastic in the frames, no v profiled rims, no threadless stems, no cupless frames (meaning it must have bottom bracket & headset cups), no tubeless tires, no odd offset pivot bolt brake calipers, no disc brakes, and definitely no bluetooth or electronic shifting.
__________________
70's Nuovo Record Jeunet Franche Compte, '88 105 Trek 1200, '85 Victory Bianchi Vittoria, '89 Exage Bianchi Strada LX, & '11 Shimano Masi Partenza