Bring broth, mushrooms and ginger to boil in large pot. Reduce heat and simmer 3 minutes. Add fish sauce, soy sauce, sesame oil and chili oil and simmer 2 minutes. Add bok choy and simmer until bok choy is tender, about 2 minutes. Stir in rice vinegar. Season soup to taste with salt and pepper.
Ladle soup into bowls. Sprinkle with green onions and serve.
*Available at Asian markets and in the Asian foods section of many supermarkets.
