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Ha. The serving staff will most likely not be aware of it and if you ask for the fish sauce to be replaced with soya sauce usually you'll get a dish without either. That's not necessarily a problem, as there will be soya and chilli sauces on the table, but it will probably be vegetables cooked without any spicing (the only flavour is often the fish sauce) and won't be very tasty--at least that's been my experience four times out of the last five. Hopefully, it'll be a place with more unusual veggie dishes that don't follow that simple formula!
Careful in Vietnamese restaurants, though, in which usually the entire 'vegetarian' menu consists of dishes made with fish sauce. :)