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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!
Actually going to a Vietnamese place tonight - will make sure to check (or, if I'm really hungry, just won't ask)...