You'd have to be willing to travel a bit - Google reckons 20 or so mins by bus, probably a bit less if you cabbed it - but that would take you to a road with, over a couple of hundred metres or so, probably the best Vietnamese restaurants in the country. It's a bit like Chinatown, in that there must be seven or eight cheek by jowl, but you really can't go wrong. The food is superb (always assuming you like that sort of thing) and the prices very cheap.
The stretch of road I mean is just below (south of) Geffrye Museum. Any cabby would know it. Or just get to the Geffrye Museum, stay on the same footpath and head back toward the river (ie, look at the museum, wheel to your right, and walk) and within a minute you'll be passing tthe first one.