Cover rice sticks with warm water and let stand for 30 minutes. Combine stock, water, onion, fish sauce, salt, pepper, red pepper flakes, lemon juice and lemon grass. Simmer for 15 minutes. Drain rice sticks. Add them and the chicken to the soup. Simmer for 10 minutes. Place a handful of bean sprouts in the bottom of a soup bowl, ladle in the hot soup, sprinkle with scallions and cilantro and serve.