Fantastic little restaurant run by a family of deaf people. Go for the Banh Khoai - crepes with fish and/or veg - but make sure you leave with one of their trademark bottle openers - handmade from a piece of word with a bolt on it. The owner will make, sign and date one for you. Amazingly effective and a memorable souvenir.
The atmosphere is really good, the staff friendly, the food is great and very cheap - a meal for less than £2. Good for veggie food too. Beware the imitators, including the place next door!
Lac Thien: 6 Dinh Tien Hoang. West of the river, near the Citadel.
I've eaten at both and they're as good as each other in every way. Plus, if you do eat at both you get two bottle openers.