I rely heavily on _Real Vegetarian Thai_ by Nancie McDermott. She says
To replace fish sauce, I arrived at a simple formula of increasing the
salt while adding vegetable broth and a
little soy sauce. In some cases, where an especially
pungent, over-the-top saltiness in required, a touch
of Asian bean sauce can be included.
According to her, fish sauce is a seasoning.
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