This large restaurant (or you could even possibly term it a dining hall, though only due to the size) is a short walk from the intersection of Cong Quynh, Pham Ngu Lao and Nguyen Trai streets. We paid 260,000VND per person for a buffet dinner, and we seemed to be the only non-Vietnamese people in there. I take this as a sure-fire sign of the popularity of the place amongst locals, and I think it's a hidden gem that no package tour groups have or will invade. The place doesn't need special kickbacks and deals to keep it running, it sells itself on its own merits.
They open for Lunch from 10am till 2pm, and Dinner between 5pm and 9pm according to the business card. My suggestion is to get in earlier rather than later, as the buffet didn't seem to be exactly my idea of a buffet, in that they stopped re-stocking the food at least an hour before closing time, so I was left with not so big a variety to choose from as I came in relatively late.
The place has a big selection of crabs and prawns that you bring to your table and cook yourself in a cooker in a great spicy broth. Just make sure you get a cooker, I asked some of the wait staff and they brought me one, but they looked to be in short supply, and you may have to share a large table with other people, but that can't be too bad! Alongside the seafood they have fresh vegies, mushrooms and noodles. In the middle of the restaurant they have a buffet selection of other Vietnamese dishes. I missed out on many of these, but like I said, get in early! Along another wall they have desserts/fruits, as well as grilled seafood, and along the opposite wall they had cold water and iced tea.
Tran Ngoc Thuy really surprised me after all the restaurants I'd been to in Vietnam. They are relatively expensive by Vietnamese standards, but still great value for money by western standards, and that also suggests that it's a place for "middle to upper class" locals, so the food is good. I'd definitely go back, and I'd be there early!