It is probably a moving target.
One definition might be a bike or frame that is out of production, and has a number of features that are substantially different than current production models.
That becomes more complicated when one considers several companies that are either making "reproduction" bikes, or continuing to work in steel.
I tend to consider my Colnago to be vintage.
It has a mix of new and old parts, and some custom parts I've built up. However, the frame has been out of production for about 20 years, and has features that have been largely abandoned by companies including a horizontal top tube and tube and lugged carbon fiber construction (although Colnago still may do that with their C60 frames). Tube sizes and shapes also more resemble the old steel frames than the new CF frames.
Nonetheless, the interpretation is in the eyes of the beholder.