There are some things about vintage watches that you need to be aware of.
The farther back you go, the less likely your watch is to have decent water resistance. Modern rubber and sealing technologies are what make water resistance so ubiquitous. So if you expect to do more than just splash some water in the general direction of your watch while washing your hands, be careful what you buy. Even going out in the rain can be a hazard with some watches. If the watch is supposed to be water resistant, make sure that they've tested the seals and are able to tell you what the results were. If not, find out from the watchmaker what he or she would recommend as far as water exposure is concerned.
Believe it or not, many vintage watches use acrylic crystals. Back then acrylic was seen as high tech and cutting edge. It scratches easily, but can be polished out and replacement crystals are generally available. You're not going to find a vintage watch with a sapphire crystal.
Vintage watches, even if they are properly serviced, don't keep the same time that a modern automatic or hand wind watch does. That is not to say that they are awful, but don't expect the +/- 10 seconds per day that an ETA 2892
will commonly provide.
Modern movements are more shock resistant than most vintage movements, so be extra careful about dropping it.
Edited by LeeReynolds - 7/22/11 at 4:35pm