Of all the places I have traveled, Vietnam was one of the easiest for finding gluten free food. Generally speaking, they have few naturally wheat-based dishes, so you really only have to be mindful of dishes that have soy sauce, hoisin, or oyster sauce in them.
For breakfast or lunch, pho is an excellent option. It may seem strange for a Westerner to eat a big bowl of hot soup for breakfast (especially in a hot climate) but this really starts your day off right. It’s is light, hydrating, and almost always gluten free. You can have a variety of meats or veggies and delicious broth with rice noodles.
Bun (or noodle bowls) are also great for gluten free travelers. Look for grilled meats (not meatballs) and veggies on rice noodles with fish sauce – delicious. Stir-fried meats on rice are also usually gluten free, just watch out for brown sauces.
Snacks such as recognizable brand chips and chocolate bars are widely available as well. Look for Snickers, Toblerone, and lays chips. And definitely, enjoy some amazing Vietnamese coffee with condensed milk for a caffeine pick-me-up!
Language cards always help overcome the language barrier. Find some gluten free Vietnamese cards here: http://www.celiactravel.com/cards/vietnamese/
Enjoy the delicious food of Vietnam and safe travels!