has to be the Coconut Restaurant - incredibly BAD FOOD! My papaya salad tasted like sauerkraut with fish sauce, and my Cantonese noodles looked more like an Italian pasta dish that was badly overcooked. Never again!
I intend to eat again at the Night Market Buffet because the food was fresh and tasty (not to mention cheap and healthy).
Try any street stall that attempts western dishes, especially sandwich/burger bars in late afternoon when the mayo or pre-cooked egg has sat around all day. Potential quick weight-loss program when you get food poisoning and can't hold food down for 3 or 4 days whilst constantly leaking water from 2 or more orifices. Actually the worst I had in Laos was a coconut pancake, maybe due to the rancid margarine or whatever had been cooked in the same pan that day.
I'm back here in Hanoi now, and really miss that good Swedish pizza I had at Scandinavian Bakery. Seems to me that Vietnamese standards of cleanliness are also pretty poor. For instance, my 2* hotel here has this buffet breakfast which is pretty good, except the plates are half clean and the tables are greasy.
I tend to avoid street food period - for the reasons you cited. It's not as if you can't get sick in Asia eating Asian food. Sloppy health habits are sloppy health habits. Some western food requires a bit more preservation - although certainly not all.
Ask a local? You think I trust these guys opinions on anything Somsai? Are you kidding me? Like I said in another, unrelated post, God knows I love 'em, but Thais and I don't regard things like cuisine in quite the same manner - ditto for driving. The indigenous persons here are, well, we are definitely different in a number of areas. What they think is good, and what I think is good, are two different things. Also, yes, I speak Thai, but I speak it badly. So nuanced questions are out. For a lot of people here, good means cheap. Now, after living here a while, I know a number of people who's opinions on the subject I would be inclined to trust. But it took a long time to meet those people.