So I just went on a 2 week tour through Vietnam with a friend of mind who isn't of the exact same eating mindset but was willing to tag along. I followed mostly chowhound tips, being wary of most tourbook restaurants, and after two weeks have this to add:
1) DO NOT EAT AT RESTAURANTS - apart from two very notable exceptions, one in Hanoi and one in Ho Chi Mihn city, (Quan An Ngon - you just have to go to this place if you're in HMC. Absolutely sensational food and variety, as well as beautiful seafood.) restaurants, even restaurants suggested in chowhound and multiple other sources, just werent as good. Ever. That includes, in Hanoi, the roundly well-reviewed (and extortionate for Vietnam) Brother's Cafe, and also, in Hoi An, the equally well-reviewed Cafe des Amis. Neither were bad, per se, just not nearly as good as good street food, which is easier to spot and usually about 30 times less pricey (you can get a great full dinner of two soups for about 60 cents most of the time, and that's on the pricier side of street food.) A general exception exists to allow for those half-street-half-restaurant places (street stallish but inside) with white walls and open fronts that really are restaurants, but pretty much only for the Vietnamese.
2) You will not get sick off of street food. If you do, you'll get over it soon. I ate raw lettuce off the street with soups or sandwiches or other things maybe every other day of the trip. I even drank some tap water. Didn't even feel my stomach rumble. Eat on the street. It's perfectly safe. That said, onto the actual recommendations:
My hotel was in the old quarter on Hang Bo street, a short strip because many vietnamese streets there change names when they hit a new block. There were two fantastic street shops on the block. I, in the course of 5 days there, ate possibly 5 or 6 meals a day, in many different places, and I am certain these places are good even relative to other street food. There are clearly even better ones out there, but if you want pointers, these are good, and pretty near the lake. During the morning up until noonish, there is a shop right next to the Hoa Linh Hotel, look the hotel up to find it if you want, that serves a lovely Bun Cha, a sweet vinegar pork broth with pickled radish, charcoal roasted beef-chive patties and bacon, lettuce, shiso, sticky noodles, and fresh Thai bird chilli. Verrry good. On the closest corner to this shop is another stall open mid afternoon to what is very-late-night for Vietnam food (11ish to maybe 12.) This stall serves a great My Van Than, opaque yellow vermicelli, sweet pork broth, bean sprouts, fried pork wontons, peanuts, deep-fried shallots, bbq chicken, pig kidney, and shiso, with a side broth made with clear scallion, shrimp and dried baby mushroom broth swimming with mushrooms, baby scallions, fresh pork wontons and fried bread ladyfingery things for dipping. For fantasic Cha Ca, catfish fried with tumeric and dill, eaten with noodles, dill, peanuts, sprouts and basil (I'm forgetting and ingredient,) go to the restaurant (I know, but this is like a stall of sorts - only one dish in the restaurant) Cha Ca La Vang. I think most caddies know where it is. At least mine did, and he approved of the choice. There's supposed to be a great dog and snake market (they kill the snakes, sometimes cobras, for you and cook each part differently!!) just out of town over a bridge, but my fellow traveller wasn't so excited by the prospects of eating snake and dog, and I left Hanoi, and Vietnam, without having eaten either of those. Dammit.
I went to Dien Bien Phu, and because the sun was setting in town our crappy, bizarre hotel was miles out of town, and taxis are hard to find there, we went back to the hotel and had one of the worst dinners of my life there. Bad move. I here they have a great black sticky rice there. Terrifically nice people. We were the only westerners we saw, and everybody said hello. We even got invited into the houses of people who didn't speak a word of English and given tea.
I wasn't a tremendous fan of the local specialty soup, Bun Bo Hue, a very lemony (citronella, actually) soup with meatballs and a shank of some meat, amoung other ingredients. The best one I found, though, was on 11 Ly Thoung Kiet. Worth getting if you have an extra meal. The unmissable meal in Hue is in one of those indoor stall places, called Hang Me. It's on 45 Vo Thi Sau. If you've already heard about this place, note the address. It's changed recently. Wow. We got one of everything they served (5 local rice-flour-shrimp specialties), for lunch. Really great, really cheap. Not a white guy in sight. Just go there and order everything. It's not too much. It's sort of tapas.
Our first dinner was a late one on the beach. It was great. I don't know the place, but he took us back to see all the seafood swim before cooking us a 6 or so course seafood meal with lots of drinks served to us on lounge chairs on the beach for a total of about 20 US (expensive there, but it was pretty worth it.) I assume there are a whole bunch of like restaurants on the Cua Dai beach. We also ate at Cafe des Amis, a place now pretty famous on Chowhound, that was OK. You just tell him seafood meat or veggie, and he gives you five courses. Nice, delicate and all that, just not local, and at least for a couple courses in our meal, not as good. It's still a good reastuarant, though, so don't be terribly discouraged. We also ate, on our tourbook's encouragement, at this weird, empty place called Faifoo, which is to be avoided, even though the waitress was nice. The place across the street looked better - more popular with locals.
QUAN AN NGON. Amazing place. Some sweet dude found all the best vendors in the city and paid them to cook there. Great seafood, loads of fantastic dishes. Great place. Really. I went there twice on my last day. There was also a nice stall on Bui Vien street (sorry for the vagueness, but you can find it) that served fantastic seafood and a mean half-formed duck embryo, if you're into that sort of thing.
Have fun. The food's great. Eat everything. Eat fruit too (try manosteen - called something like mank(r?)oot, - they're great.) And the coffee, wherever there's a Trung Nguyen sign - Vietnamese Starbucks with better but sweeter (sweetend condensed milk with STRONG drip espresso.)
PS - if you're in Hoi An and want tailored clothes, DO NOT go into the market. Period. No exceptions. Market people will try to convince you otherwise. It's BS. the two places you should consider are Mr Xe (71 Nguyen Thai Hoc St - specializes in men's suits but does other stuff too, and it's worth going there just to meet him) and Bi Bi Silk (13 Phan Chu Trinh St. - most things, including gorgeous silk shirts and a nice variety of linen.) Have a great trip if you go.