Combine first 4 ingredients in a small bowl, stirring until well blended. Cover and chill.
Place steak between 2 sheets of heavy-duty plastic wrap, and pound to 1/4-inch thickness using a meat mallet or small heavy skillet. Combine pepper, vinegar, salt, and garlic in a small bowl. Rub steak with vinegar mixture.
Place steak on grill rack coated with cooking spray; grill 2 minutes on each side or until desired degree of doneness. Cut steak diagonally across grain into thin slices.