For a soup that's not strictly vegetarian but with more Southeast Asian flavor, replace 3 tablespoons of soy sauce with an equal amount of fish sauce.Plus: More Soup Recipes and Tips
Bring 8 cups of water to a boil. Remove from the heat, add the rice noodles and let soak until the noodles are softened, about 5 minutes. Drain and transfer to a large bowl.
Heat the oil in a large saucepan. Add the ginger and lemongrass and cook over moderately low heat, stirring, until fragrant, about 2 minutes. Add the remaining 6 cups of water, cover and bring to a boil over high heat. Add the sweet potatoes, soy sauce and crushed pepper and season with salt. Lower the heat, cover and simmer until the potatoes are tender, 7 to 10 minutes.
Add the cabbage and coconut milk and simmer, uncovered, for 5 minutes. Stir in the lime juice and cilantro. Add the noodles, ladle the soup into bowls and serve with the lime wedges.