Famous (mainly due to Anthony Bourdain) for her rotating menu of 7 different soups, one for each day of the week. I went on a Saturday and had a delicious Bánh canh cua (a fish soup with crab and thick noodles - 50,000d), then went back on a Tuesday and was disappointed by a quite boring soup (40,000d). The outdoor seating in the shade of some big trees adds to the great atmosphere with a mix of locals and tourists. The Lunch Lady is very smiley and she happily posed for a photo.
Note: They will bring you plates of other snacks and place them on the table as if they're included. They're not. This is a sneaky tactic that you sometimes see around Vietnam and I don't like it at all.