Thaw fish, if frozen. Rinse fish and pat dry with paper towels. If necessary, cut fish into 4 serving-size pieces. Set aside.
For glaze, in a small bowl combine brown sugar, soy, and mustard. Set aside 2 tablespoons of the mixture for sauce.
Preheat broiler. Place fish on the greased unheated rack of a broiler pan.
Broil about 4 inches from the heat for 8 to 12 minutes or until fish flakes easily when tested with a fork, carefully turning once halfway through broiling and brushing with glaze the last 2 to 3 minutes of broiling.
For sauce, stir together rice vinegar and reserved brown sugar mixture until sugar is dissolved. Serve with salmon.