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.
Preheat a 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. (Alternatively, grill in batches on a lightly oiled ridged grill pan over medium-high heat on stove.)
Transfer steak to cutting board, tent loosely with foil and let stand 5 minutes before slicing and serving.