Au Lac is a small, corner restaurant that serves to an almost exclusively local crowd. The food is really good, especially for the price. During the day they they have a great buffet of mock meat, stir fries, rice, spring rolls, etc. The staff is very friendly and don't speak English but you can get by by pointing at what you'd like. You can also order off the menu but it's written entirely in Vietnamese so bring a goodphrase book (the Lonely Planet phrasebook has few translations). The pho on the menu is fantastic. Two people can eat here, ordering pho and Vietnamese ice tea (tra da) costs around 36,000 dong/ a little more that $2 USD. I have not been there for dinner but imagine its pretty similar.
Cons: language constraints can cause issues