In the end the idea is that the risk is not worth: the potential damage is very huge and we know how much these vintage calibres, dials etc. now cost.
And for some you can't even think of getting them no matter how much you'd be ready to pay.

The real issue is in the seals and gaskets: modern watches have ones that are produced now with today's technologies and recent materials.
Vintage watches typically use older ones and even NOS supplies the very age of the components matters.
And a little tad of humidity can do more damage than you'd expect - i.e. trigger oxydation processes leading to rust or worse.
And for some - e.g. the Monaco's -some even doubt they really ever were waterproof. It's the same as with vintage cars: you seldom see them out in rainy days, do you?

Just for your knowledge, over the years I've had some of my vintage watches (i.e. older than 30 years) overhauled by their official makers: well, for the older ones some did not pass the test even after the service (and this waterproofnes is not guaranteed in fact). But the most startling find was that even for those that did pass with flying colours, the maker always enclosed a letter that said "Despite the substituion of this and that and the positive outcome of the pressure test, given the age of the watch we do not guarantee etc. etc."
So really at your own risk.
In the end: wear a beater if you want, but not one of your most loved or precious one.
Cheers,
Fabrizio