Slice the bulb end of each lemongrass stalk diagonally into pieces about 1/8 inch thick. Peel off four wide strips of lime rind with a potato peeler, avoid the white pith underneath, so it does not make soup bitter. Squeeze the limes and reserve the juice.
Add the sliced lemongrass, lime rind, chiles, galangal and cilantro stalks to the stock, with the kaffir lime leaves, if using. Simmer for 1-2 minutes.
Add the monkfish, rice vinegar and fish sauce, with half the reserved lime juice. Simmer for 3 minutes or until the fish is cooked. Lift out and discard the cilantro stalks, taste the broth and adjust lime juice if necessary.