To make Broth: Bring vegetable broth and 5 cups water to a boil in electric wok or electric skillet over medium-high heat. Add lemongrass, cilantro, ginger, garlic, chiles, soy sauce, brown sugar, and vinegar. Reduce heat to low, and simmer 30 minutes. Remove lemongrass, ginger, garlic, and chilies with slotted spoon, and discard. Place broth in center of table for communal dipping.Let guests dip vegetables, potatoes, and dumplings in simmering broth to cook. (Most vegetables take 2 to 3 minutes, potatoes and dumplings 5 minutes.) Serve with sauces.To make Soup: When all the dippers have been cooked, add rice noodles and sprouts to broth (any uneaten dippers can remain in the soup). Simmer 5 minutes, or until noodles are tender. Serve garnished with onions, mint, cilantro, and peanuts from Lettuce Wrap garnishes. Add hoisin and/or chile sauces, if desired.