If using dried rice vermicelli, place it in a bowl and cover with lukewarm water. Let stand until pliable, about 20 minutes. Bring a pot of hot water to a boil. Drain and divide the rice vermicelli into 4 equal portions. Place 1 portion at a time in a sieve and lower it into the boiling water. Untangle the noodles with chopsticks and boil until tender but firm, about 3 seconds. Remove, drain, and place in a large bowl. Repeat this step until you have 4 individual servings. If using fresh bun, there is no need to boil them.
Divide the noodles among 4 bowls. Scatter some lettuce, carrot, cucumber, and mint over each bowl. Drizzle with nuoc cham and toss well.
Fish Dipping Sauce: Nuoc Cham
Whisk together the sugar, water, fish sauce, and lime juice in a bowl until the sugar is completely dissolved. Add the garlic, chile, and shallot, and let stand for 30 minutes before serving.
Recipe courtesy of Corinne Trang, Authentic Vietnamese Cooking, Simon & Schuster, 1999