Hu tieu Nam Vang is one of the lighter noodle soups in Saigon at about 400 calories, and it is a decent protein bowl because of the mixed toppings. The 180g of hu tieu noodles is 200 calories. The toppings are a spoon of minced pork, three or four slices of lean pork, one or two prawns, a slice of liver and a quail egg, about 90g together and 140 calories. The broth is clear pork bone stock, around 50 for 300ml.
The version matters. Vinmec lists plain hu tieu thit heo at 351 calories, Nam Vang at 400, hu tieu mi (with egg noodles mixed in) at 410, hu tieu bo kho at 538 and hu tieu xao (stir fried) at 646. The dry hu tieu kho is not on their list but the dressing of dark soy, sugar, pork fat and fried garlic adds about 80 calories over the soup version.
The thing that makes hu tieu bigger than the number suggests is the free bowl of broth that comes with the dry version and the pile of fried garlic in oil on top. At a busy quan the topping bowl of toi phi (fried garlic) sits on the table and people spoon it on freely. One tablespoon is around 50 calories. Two spoons of that plus a squeeze of chili sauce and the 400 calorie bowl is 520.