-If you want to make this truly vegetarian/vegan, you can leave out the fish sauce—just be sure to add extra lime juice and salt to make up for loosing out of the salty sourness the fish sauce would bring. I’ve heard that some people use Worcestershire sauce as a fish sauce substitute but, for one, there’s anchovy in Worcestershire so that defeats your purpose, and two, I just can’t see the two as being interchangeable. There may be a worthy application where this substitution is appropriate but I really don’t think that it’s here.
-To make this gluten free, make sure the brand of stock and fish sauce are gluten free. Most fish sauces are, and so are a lot of stocks, but just double check. Obviously, the naan is out of the questions unless you can find a gluten free option (made out of garbanzo flour or something). You could always cook up some white rice to serve alongside the soup and everyone can just add it to their own bowls to make this a little more filling.