RSC service of vintage unit?

I wouldn't say RSC are any more or less likely to get a vintage water water resistant than a good independent watchmaker. It also depends more on condition than age. If there's pitting/corrosion around the crystal/caseback seals it can be very difficult to restore any real world water resistance, short of replacing the case. Obviously this is where RSC come in as they would be able to supply a new case. However, for an 80's OP/Date, the cost wouldn't make financial sense unless it had sentimental value.

The main thing is to know what you're looking for and try to find a good example in the first place. Perhaps one that's already been serviced by a reputable watchmaker and has passed a water resistance test.

RSC is a grand or more. I think they accept 80s watches. You will need to check. I’d have no issue swimming with a watch under their warranty. A good local watchmaker would be approximately half that, but you would need to do your homework.

Rolex are sealed a lot better than others, but is it really worth taking vintage ones it in the water? I think recently serviced watches are OK, but 904 steel Rolex are the best because there is less pitting to the cases.

An RSC will generally accept most 1980s watches, at least for now. Whether it will pass a pressure test depends on the condition of the watch. Try to buy a watch where you can see the caseback and the back of the mid-case with the gasket removed. A small amount of corrosion/pitting could still be ok, but not too much.