I just came back from a moto tour this past January and discovered 3 great dishes.
If youre in Hanoi, you gotta try Bun Cha. It's charbroiled ground pork patty, lettuce, herbs, rice noodles and you drown them in a fish type sauce. Its excellent.
Then you gotta try Xoi Xeo. It's sticky rice, with fried mung beans, shredded chicken all covered in fried shallots. It's so great, Im willing to bend my bad knees sitting on little red plastic chairs.
3rd, if youre with company (or really really hungry) sweet and sour hotpot is AWESOME.