It has been a while. It has been so long that Hanoi seems like a dream now... I did find some allergy-friendly restaurants in Hanoi:
Bobby Chinn77 Xuan Dieu Street, Tay Ho District, Hanoi
In case you don't know, Bobby Chinn is a celebrity chef with his own show on TV. We were lucky to meet him in person at the restaurant that evening. When I emailed to make my reservation, I told them about my allergy including the "gluten-free" part and they completely understood. The waitress even told me they substituted the flour in the turkey gravy with rice flour. The Christmas dinner costed ~70USD including champagne. It was well worth the money... let's forget the Vietnam price level for a while. It is a bit out of the way but Bobby Chinn was saying that they are looking to move back to the old town area.
Cozy ambiance perfect for a champagne.

My cold white bean soup

"squid pasta"... flour-free pasta made with squid

Heavenly mushroom risotto - always my fav dish

buckwheat pancake with smoke salmon and cavier (with gluten)

my gluten free alternative: cheakpea pancake bruschetta

dessert!
48 Hang Be Street, Hoan Kiem District, Hanoi
Vietnamese style French cuisine. Yummy and creative dishes. The Ca qua fish in tamarind sauce is really good. As well as the dish in the phone below... Email them in advance for arrangement for allergy. (I didn't specifically ask for GF food, but instead just picked dishes that look ok.)
MSG-free restaurant. I didn't request gluten-free but I'm sure they can arrange something if you talk to them in advance.
We were at Ms Chang's ecological home stay. I told my guide about my allergy when booking the trip and on the tour. I ate most but not all of the dishes. I didn't get any reaction from the trip... It was this fantastic tour made me fall in love with Vietnam.
Street foods I didn't try

Pho - 49 Bat Dan. Long line up til late at night. I took a bite of the rice noodles, yummy!

Eel noodles - 87 Hang Dieu Street (the stir fried one is the best according to my friends. But I didnt touch the food cuz I saw huge packages of MSG on the shelf)

Ok, I took a few bites of this rice rolls. Yummy! Even in HK/ China, you can't easily find freshly made and steamed rice rolls like this! 14 Hang Ga Street

Vietnamese bun. Obviously gluten filled. I didn't touch...
Special thanks to the Canadian Chinese family we met on the Halong Bay trip for this street food cheat sheet (original source:
gastronomyblog). I didn't try everything on the list. But for those my friends tried, they are all good.
Good luck with your Hanoi trip!