I just spent four nights in Cypress, and ate at Thuyen Vien three of those (they're closed on Tuesdays). They were very welcoming; I got to know the owner, who was happy to discuss cooking, ingredients and his menu with me. The first night I had the roast chicken, which was a good dish for people who don't want to end up with too much food. It was a very tasty sweet sauce, with good texture. Next I tried the fish with a seaweed skin, one of their fanciest dishes. This one was an enormous portion - best for two people. It featured incredibly fresh mint, zucchini, jacima, and thin rice wraps - a Vietnamese fish tortilla as it were. Amazing. The last night, I took the recommendation of the owner and had the fish hot pot as well as the sweet and sour soup. Again, there were great herbs (basil in this case) in the soup, as well as chunks of tomato and other great vegetables - and he recommended eating the two together (fish pieces in soup).
Prices were unbeatable (from $6 for the chicken or soup to the highest-priced $14 fish wraps) for the quality and serving size.
Atmosphere was friendly, clean, and authentic. If I had any caution, it would be that they keep the door open (no A/C I could see), which can get warm (but is great for the environment). Go there - and ask them what you should have. They won't disappoint you.