I went to this place twice. The food is simple and tasty. Only locals, I saw a few monks, and cheap. 17,000 dong for a dish with rice, tasty veggies and a few delicious tofu pieces. The owner is very friendly. She is learning French and she loves to practice it. She also assures me she does not use MSG and is available to give cooking classes. Nice vibe.
I find this place by chance. The address initially posted here is wrong. Here is the address:
Bo De Quan
34 Han Thuyen
It is a few blocks from the Eastern door of the citadel.
There are two more vegetarian places on the same street but I did not try them!
Cons: Limited menu (but that could be a plus! ), Menu only in Vietnamese