I'm buying a 1971 Geneve, which were 30M water resistant. I know these vintage watches aren't typically trusted to remain water resistant, but is there any process or a set of procedures to reestablish this feature of a watch that at one point was waterproof?
Water resistant to 30M does not mean waterproof, only that it will survive occasional splashes from shower or handwashing. There is a lot of info on the net about what the depth ratings really represent. As to your watch, if the seals are good and case/crystal still tight, a test that you can have done at a good watchmaker will confirm if it is still ok. If not, it will need a service to get those elements fixed. Personally, I would never expose a vintage watch to excessive humidity risk, even if the tests show it is still pressure tight.
Save yourself a lot of money and don't wear a vintage watch underwater. Yes, it likely could be made "waterproof" again, with new factory crown, gaskets, and crystal. But that's not why you buy a vintage watch. If you're diving, buy a dive computer. If you're splashing in the pool with you kids, buy a casio, timex, or seiko... or stick to modern watches.