Hi! You can use Chinese oyster sauce or vegetarian "oyster" sauce (made with oyster mushrooms) instead of the fish sauce. 2 tablespoons of the stuff diluted in 2 tablespoons of water works the charm.

Oh, and I've tried doing a vegetarian version, too. (Soft tofu + vegemeat = nice!) If you're a lacto-ovo vegetarian, adding hard-boiled eggs before serving is also a good idea.