Broth: Place 2 cartons of vegetable broth and water in a large pot. Bring to a boil.
Heat vegetable oil in a large sauce pan, add minced lemongrass, leek, chili powder, Hue seasoning (if using) and lemongrass stalk. Saute for 2 minutes, until fragrant. Transfer the mixture to the broth.
Toppings: add a bit more oil to the sauce pan. Add in chili oil. Toss in mushrooms, tofu and season to taste. Transfer the mixture to the soup pot.
Rice noodles: bring a big pot of water and cook noodles according to package instruction.
To serve: transfer some rice noodles to serving bowl. Ladle some broth over, with toppings. Serve with fresh herbs, bean sprouts and more chili oil on the side. Enjoy!