nha hang ngon, located in three cities--Saigon, Ha Noi, and Phnom Penh--serves traditional Vietnamese food from all regions. at least the one in Saigon does, and its book of a menu makes that part clear.
with various prep stations and kitchens situated against the left, right, and back walls--it's worth taking a look & walk throughout this two-story-villa-now-restaurant. always crowded but with a high circulation and airy ventilation, this spot, frequented by both locals and foreigners alike, is a reliable source to get either a cursory or thorough overview of a wide breadth of Vietnamese dishes.
and if you plan to be in the area, drop in on a class ($6.00 USD) at yoga living (95 Pasteur) right across the street. (also attracts a mix of locals and foreigners.)
or--right outside the restaurant--try to catch the gentleman purveying kilos and kilos of dark ruby-toned mangosteens from his motorbike.