Sprinkle cracked pepper evenly over both sides of each salmon steak, and place steaks in a 13 x 9-inch baking dish. Combine vinegar and next 6 ingredients (vinegar through garlic) in a small bowl; stir well. Pour vinegar mixture over steaks; cover and marinate in refrigerator 1 hour, turning steaks occasionally.
Prepare grill. Remove steaks from dish, reserving marinade. Place steaks on grill rack coated with cooking spray, and grill 5 minutes on each side, basting frequently with half of reserved marinade. Combine remaining half of marinade and cornstarch in a small saucepan; bring to a boil, and cook 1 minute or until thickened, stirring constantly with a wire whisk. Spoon about 1 tablespoon sauce over each steak.