Combine first 3 ingredients in a small bowl; stir with a whisk. Reserve 1/4 cup vinegar mixture.
Combine red pepper, salt, and five-spice powder; rub evenly over skinned side of salmon.
Place salmon, skin side down, on a broiler pan coated with cooking spray. Broil 16 minutes or until fish flakes easily when tested with a fork or until desired degree of doneness, brushing frequently with vinegar mixture.
Place reserved 1/4 cup vinegar mixture in a small microwave-safe glass bowl. Microwave at high 20 seconds or until bubbly. Drizzle over fish; sprinkle with onions.