I think the OPs question was more how does vintage frame sizing relate to modern frame sizing. And the answer is huh?! There's no frame sizing standard. A labelled 56cm frame, be it modern, vintage or otherwise can have a myriad of other measurements, i.e., top tube, head tube, etc. Heck even the 56cm measurement may be taken differently....some mfgrs use CTC some use CTT for the measurement.

The only right think to do is to measure for yourself a frame you feel fits you and find another frame with roughly the same measurements. Then, use stems and bars to dial in reach and drop and seatposts to dial in saddle fore-aft.

OP, there's no standard, so there's no answer to your question.