Dear Splodger, I hope that someone else can come up with better news for you, but based on my own experiences waterproofness is very seldom if ever guaranteed again for a vintage watch, no matter who's doing the service.
In fact even in the fortunate case where the service people are able to find all the right pieces (gaskets, seals etc.) they don't want the burden of guaranteeing: this means the watch may actually be waterproof (and usually is), but no explicit or formal statement will be provided.
Quite the contrary indeed: often, though not always, they'll provide you with a statement that will say something like: "Although all relevant parts were substituted, the very age of the watch prevents us from providing you any warranty for it being waterproof" etc.
Again from personal experience I have always taken this with common sense, and I've worn my watches w/o paying attention to this: of course I don't take a shower or a swim with them but no problems with everyday activity like washing your hands or getting caught out in the rain.
Hope this may somehow relieve your worries.
Fabrizio