Combine vinegar and steak in a large zip-top plastic bag. Seal bag, and marinate in refrigerator 30 minutes.
While steak marinates, place potatoes in a saucepan and cover with water; bring to a boil. Reduce heat; simmer 15 minutes or until tender; drain. Return potatoes to pan. Add buttermilk, butter, chives, and 3/4 teaspoon salt; beat with a mixer at medium speed until smooth. Keep warm.
Preheat broiler. Remove steak from bag; discard marinade. Sprinkle steak with 1/4 teaspoon salt and seasoned pepper. Place steak on broiler pan coated with cooking spray; broil 6 minutes on each side or until desired degree of doneness. Cut steak diagonally across the grain into thin slices.