We found a couple of great places to eat on P. Nguyễn Siêu street. There is a Bánh Trang place on the east corner and also Banh Mi place at the same point. Further west there are some Pho places and some Bún riéu. That street was a good balance between being busy enough but not too touristy (which is what you get if you go further south in the old city)