1. Combine vinegar, sugar, rosemary and 1 1/2 tsp. salt in a ziplock bag. Add steak to bag, seal and turn bag several times to coat meat with marinade. Refrigerate 20 minutes, turning once or twice.
2. Preheat gas grill to high for 20 minutes. Remove steak from marinade and sprinkle both sides with salt and pepper. Discard marinade. Grill steak for 4 to 6 minutes, turn over, and continue to grill until done, 3 to 4 minutes more for medium-rare, depending on thickness of meat and heat of grill.
3. Transfer steak to cutting board, tent loosely with foil and let stand 5 minutes before slicing and serving.