One thing that can make eating out at Southeast Asian restaurants challenging for vegans is that they use fish sauce in almost everything. Luckily, the Vietnamese often do Buddhist fasts and so they have come up with some great ways to get around the prevalent use of fish sauce. In some specialty markets you can find veg fish sauce which I often call for in recipes that I post. But you can always use more soy sauce too and sometimes I use Oyster (mushroom) sauce as well. If you want to make your own fish sauce, here is a recipe from Hot, Sour, Salty, Sweet: A Culinary Journey Through Southeast Asia which is one of my favorite cookbooks. 
It isn’t at all fishy but it adds some complexity to Thai recipes and a little bit of that Southeast Asian flair.
Mix
3 Tablespoon soy sauce
1 Tablespoon rice wine (or rice vinegar)
1 teaspoon garlic, chopped
1 teaspoon lemongrass, chopped
1/8 teaspoon cinnamon
1 bird chili (or other hot little pepper or Sriracha)
1 teaspoon peanut oil
1 teaspoon sugar
1/2 teaspoon salt
The key, of course, is to have really good soy sauce. Usually I have several different kinds that I get when I go to the giant Asian Super Market a couple times a year. I like to have both light and dark and I often use both in the same recipe. It doesn’t hurt to try different kinds!
In truly lazy style I recently boiled some soba noodles, added some bok choi, drained and poured the whole recipe of fish sauce into a pot and mixed it with the noodles and greens and a little bit of oil. It was good enough to eat, cheaper than take out, and even healthier. Looking at those noodles is making me so hungry