BRING stock and vinegar to boil in stockpot on medium-high heat. Reduce heat to low. Stir in lemongrass, galangal, kaffir lime leaves and chili paste; simmer 5 minutes.
MIX lime juice, fish sauce and sugar in small bowl until sugar is dissolved. Add to stockpot with prawns and mushrooms; simmer 2 to 3 minutes or just until prawns turn pink.
REMOVE lemongrass and kaffir lime leaves before serving. Stir in chopped cilantro and chiles, if desired. Ladle into bowls. Garnish with sprigs of cilantro.