It's in the 7'th, and so the prices are understandably higher, but this remains one of the best Vietnamese places I've tried in Paris. I recommend the Bo Bun as a meal and the Perles de Cocos for dessert. They sometimes do a traditional "Pho" soup (beef noodle) which is exceptional. It may not be on the menu, but it's worth asking.
The owner is a bit surly, and while he does make up for this by wearing a bow tie, don't expect a traditional obsequious American greeting, a la "Hi, I'm Brittney, I'll be your waitress today, have you guys been here before?...let me know if you need any help chewing your food!..." etc.. The rest of the staff is quite pleasant.
Incidentally, the restaurant is frequented by Chirac and numerous other politicians, as it's not far from Assemblée Nationale. In fact, the walls are adorned with photos of all kinds of important- looking elderly chopstick-wielding white men in suits. I've been assured by authentic French people that these men are, in fact, dignitaries.