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 to redistribute marinade.
2. Preheat a gas grill to high. Remove steak from marinade and sprinkle both sides with salt and pepper. Discard leftover marinade. Grill steak for 4 to 6 minutes, carefully turn over, and continue to grill until meat reaches desired doneness, 3 to 4 minutes more for medium-rare, depending on thickness of meat and heat of grill. (Alternatively, grill steak on a lightly oiled ridged grill pan over medium-high heat on stove, for same amount of time.)
3. Transfer steak to a cutting board, tent loosely with foil and let stand 5 minutes. Slice steak thinly against grain and serve.