You should be able to find a vintage bike at or around 50 cm. I currently have two under 50cm and I would guess they are too small for you. I just sold a 50cm Raleigh to someone who is about 5'6''. For that individual, it was a good fit. Earlier this year I met a young lady about 5'4" and sold her a Ross 290S (yeah the classic!) that was 49cm. All too small for me at 5'9". The key to finding a vintage bike of a particular size is hard searching at yard sales, thrift shops, ebay and craigslists. While they may not be common, the person trying to sell it has a real problem, the buyers market for small road bikes is small.
I think its interesting that you're having difficulty finding smaller frame road bikes because it seems all I can find are small or large road bikes. Medium (52-56cm) road bikes just don't seem to come my way. But after persistant searching at local yard sales and craigslist I now have a few.
EDIT: IMHO you should take into account the length of your inseam when sizing a bike. I'm looking for smaller frames because my inseam is only 29.5", somewhat short for a guy 5'9".