The short answer
Our advice with a vintage watch is simple: keep it away from water.
A watch may originally have been described as water resistant, waterproof or even intended for diving, but decades later that shouldn't be treated as a guarantee of its ability to resist water today.
A vintage watch has survived this long. There is very little to be gained by testing its luck.
From the bench
Water resistance isn't provided by one component. It depends upon a system of seals around areas such as the caseback, crown and crystal.
Those components age. Gaskets harden and compress, crowns wear, case surfaces acquire marks and tolerances change over decades of use.
Even where gaskets have been replaced and everything appears mechanically sound, we don't believe that turns a vintage watch back into the water-resistant watch it may have been when it left the factory fifty years ago.
Moisture and a mechanical movement are particularly unhappy companions. Once water gets inside a watch it can affect the dial and hands, encourage corrosion and contaminate the lubricants within the movement.
The damage can also continue long after the obvious moisture has disappeared.
What we recommend
Don't swim, shower or bathe while wearing a vintage watch.
Avoid deliberately running it under a tap when cleaning it, and remove it before doing anything where significant contact with water is likely.
Normal life doesn't require panic. A few spots of rain aren't the same thing as immersing a watch. Simply use sensible mechanical sympathy and remember that you're wearing something which may have been made several decades ago.
If you ever notice condensation or moisture beneath the crystal, remove the watch from your wrist and have it inspected rather than waiting to see whether it disappears.
Water resistance is replaceable. Vintage dials, movements and history often aren't.