does anyone know of a vegan substitute for fish sauce?

Thin soy sauce is a pretty perfect substitution. I use Dragonfly brand: http://www.templeofthai.com/food/sauces/thinsoy-5133231037.php

worcestershire sauce has similar taste profile. here's one commercial and one home made version
http://www.edwardandsons.com/thewizards_info.itml
http://www.marthastewart.com/258132/homemade-vegan-worcestershire-sauce
And/or if can't find vegan worcestershire in store nor want to make it, good tamari or soy, for similar not-identical umami flavor hit.

Here are two versions. I had a vegan friend over and made the one that called for seaweed since it has an oceaney (made up word) flavor. I used kombu since I always have it on hand.
http://www.thekitchn.com/recipe-vegan-fish-sauce-130535

http://veganmiam.com/recipes/vegan-fish-sauce