I am 6'3" and I ride a 25" Miyata frame. After an absurd amount of reading and fidgeting (three stems, two seatposts, two sets of bars), I have dialed it in so that it is very comfortable. However, I will buy a larger frame one day because it is about 1" or so too small.

A few things that I found out.
1. Measuring PBH (pubic bone height) per the instructions on the Rivendell site was actually pretty useful. It allowed me to calculate the proper seat height off of the center of the bottom bracket. I lowered it just a touch over time, but it was pretty spot on.
2. You can get modern mountain bike seatposts that can put your seat at the right height for basically any frame. You might have trouble with the fore-aft positioning, but this is likely solvable. Unfortunately, vintage seatposts are more limited in their minimum insertion. Pretty much a fistful above the insert line and you are at the limit. If you are gonzo over old Campy or SR or whatever, then you may want a frame that doesn't demand 6" of seatpost out of the frame.
3. The real pisser is the stem. If you get the seat too high off the frame, then you get some unattractive options for stems that have enough rise. I like having my bars just a tad below the seat, but Grant P and others say even with the seat. With a 25" (63cm) frame, that meant that I needed about 5" of stem above the headset. That meant I could either use the Nitto Technomic or the Nitto Dirt Drop (I bought both). In the end, the Dirt Drop functions better, but it just looks strange on a vintage bike. The Technomic, however, can get comical. The more you expose the stem, the further back it pulls the bars. In order to get proper reach, you have to get a relatively long model. At 5.25" exposed stem and 13cm stem size, it looked like I had brazed an anemic cobra onto the frame. Further, with wide bars, it had all of the torsional stability of cold pasta. I decided to just go with the Nitto Technomic Deluxe, lower the bars, and do more pilates.

Under no circumstances would I buy an undersized frame that is not completely standard at least by early 1980's standards (don't buy the French bike). You don't need a threadless steerer, but at least have an English threaded BB and a standard headset. It is just too hard and too expensive to buy big-guy parts for freaky threadings and diameters.

None of this may apply to you. I had a 94 cm PBH, and it means I need a big bike. But you should take it seriously. A cool vintage bike that you don't ride is really just a novelty.

Matt