Boil beef (with bones) or oxtails in 3 qts cold water. Skim off foam and fat. Cover and simmer for 4 hours. Broil onion until flesh is soft, turning often. Peel. Add onion, ginger, anise, salt, and fish sauce to beef broth just before it has finished simmering. Also just before beef broth finishes simmering, drop vermicelli into a pot of boiling water. Cook 8 minutes, remove from water, rinse in cold water, and drain. Divide vermicelli into three equal portions and place in 3 individual serving bowls. Divide beef into 3 equal portions and place on top of vermicelli in the bowls. Garnish each bowl with scallions and 2 or 3 fresh cilantro sprigs. Strain broth, reserving beef and flavorings. Pour one cup broth over contents of each bowl. Sprinkle with pepper and chiles. Serve immediately. Reserved beef, flavorings, and extra broth can be eaten separately.