David Thompson, author of the superlative
Thai Food cookbook, has this to say:
Chinese or Japanese soy sauce should not be used as substitutes–although light soy sauce can be used as a vegetarian alternative to fish sauce, with varying degrees of success.
I'm sure you checked, but most red curry (and green, and yellow, and masaman, for that matter) pastes I've seen have shrimp paste added to them as well.
Apparently,
vegetarian fish sauce does exist. I've never tried looking for it, but that article says Vietnamese places often carry it. Check the Argyle area.