Haven't been in a very long time, so I wouldn't be able to recommend specific places, but I can recommend specific dish.
Turmeric fish (Chả Cá Lã Vọng). This is a specialty and you can find places that only do that. There's a pan in front of you with some oil, and you put fish and dill and tumeric and cook it yourself.
DIY Fruit smoothies (Hoa Qua Dam). You can find that in the streets. They give you these cups with fruits and ice and condensed milk and you're supposed to blend everything yourself by hammering the content with your spoon. So good!
Bun Cha. That's my favorite dish. You'll see a lot of that as street food. I ate this while sitting on a stool on the side walk and it was my favorite meal ever. Some people might call this a deconstructed Pho perhaps.
Bun Bo (Bun Bo Nam Bo). Or like we say in French, Bo Bun (don't ask me why). This is probably the most popular dish of vietnam (with Pho), so get a lot of that! It's super fresh and that's mostly what I eat when I eat vietnamese food.
Pho (noodle soup). I assume everybody knows that one. But the one in Vietnam hits really differently. There's just no way to have a good Pho outside of vietnam IMO. Try the pho ga (chicken) and pho bo (beef). You can pho for every meal. Locals eat pho at 4am after partying and dip donuts in it.
Banh Cuon. This is one of the best appetizer/dish that is so hard to find outside of Vietnam. They're like super thing rice pancake with beef inside. They are usually so thin that they break when you try to eat them. Eat that with fish sauce and it's gold!