Vietnamese cuisine is one of our favorites. Having spent one year with the Vietnamese Marine Corps, I became quite familiar their food and culture. Later when we lived in North Branford CT, a VN refugee family opened a restaurant in the Branford Town Square. We got to know these folks and found that we had some mutual Vietnamese friends. The wife also wrote a cookbook.
A fish sauce 'nuoc mam', is the basis of VN cooking. Don't ask me how its made - you don't want to know. However you DO want to use this amazing sauce. A little splash on any meats or veggies adds a wonderful flavor. It is also a great marinade. If you add garlic, sugar, red pepper, and lime to nuoc mam, you will have 'nuoc cham'. It is one of the wonderful dipping suaces frequently served in VN restaurants. Its not thick and red, rather it is brown and clear, with bits of pepper, lime, and garlic suspended/floating in it.
I agree with Rana, above, about 'pho'. The soups make a fine lunch bacause they are so very light and they contain so many various items. You may find 2 or 3 meats, cut very thinly, a type of cabbage, perhaps bok choy, other greens, including lemon grass and scalions, garlic and other herbs, perhaps Vietnamese cilantro, maybe a little shired egg, and of course that ubiquitous nuoc mam. Then at the table you must add the bean sprouts, freash basil leaves, (where do they get such beautiful herbs?)and lime.
My wife does great VN pork roast, baby back ribs, and fish recipes - all with many of the above ingredients.
As with all 'good' ethnic food. Avoid the large very popular (with Americans) restaurants. They are much more likely to be mostly Chinese (American style) spin offs. Seek out the VN section of town. We have a fine "Little Vietnam" area in OK City where we eat and shop.