Any ideas for a vegetarian alternative to fish sauce? Or is it too central to thie dressing to tamper with? Thanks!

The thread that Kristen referenced has a great link to "fishless fish sauce" from the kitchn. The explanation is also helpful, in that it discusses how fish sauce hits the umami spot and there are only certain flavors that do that. Lime and salt will be tasty but won't hit umami -- as they indicate, fish is high in umami (hence the success of a small bit of fish sauce in a number of dishes) but so are fermented foods and mushrooms. If you want to get that umami hit, I'd suggest using thekitchn's recipe http://www.thekitchn.com/recipe-vegan-fish-sauce-130535

Thanks Kristen and Lynnht! I'm now recalling a Cook's Illustrated recipe I used to make which offered the suggestion of using lime juice & salt as an alternate to the fish sauce. I'm going to do it!

Soy sauce usually works as a decent vegetarian alternative for fish sauce.

It's pretty central to the flavor, but there were some great alternatives suggested in the comments thread on the original blog post: http://food52.com/blog/4857_momofukus_roasted_brussels_sprouts_with_fish_sauce_vinaigrette#comments