Prepare the rice vermicelli according to package directions and transfer to a colander to drain. Rinse well with cold running water. Drain thoroughly before using. (The noodles may be prepared up to this point 2 hours ahead of serving.)
In a small skillet heat the peanut oil over high heat and add the shallots. Cook, stirring frequently, until golden brown and crispy. Transfer to a paper lined plate to drain. Reduce the heat to low and remove all but 2 tablespoons of the oil from the pan. Add the chopped green onions to the skillet and cook, stirring constantly, until soft but do not allow to brown, about 1 minute. Transfer green onions to a small bowl and set aside to cool to room temperature.
Divide the drained vermicelli, lettuce, bean sprouts, and cucumber evenly among 6 deep soup bowls. Top with the meatballs, spearmint leaves, and cilantro. Garnish with the peanuts, crispy shallots, and sauteed green onions. Drizzle generously with Nuoc Cham Sauce and serve immediately, with chopsticks and more Nuoc Cham sauce on the side.
Yield: 6 servings