Serves 4; can be doubled for 6 to 8 (no need to double the marinade)
1. Whisk together the vinegar, the oil, the garlic and rosemary in a large dish (or use a thick, sealable plastic bag). Add the steak and turn to coat. Cover and let marinate in the refrigerator for at least 4 hours and up to 8 hours, turning every 2 hours.
2. Heat your grill to high.
3. Remove the steak from the marinade and season with salt and pepper. Grill until lightly charred and crusty, 4 to 5 minutes. (do not close lid on grill unless it is cold and/or windy) Turn steak over, reduce the heat to medium or move to a cooler part of the grill, and grill until medium-rare, 3 to 4 minutes more. *(If your steak is larger, grill for an extra 1 to 2 minutes per side, but be careful not to overcook).
4. Remove the steak to a cutting board and let rest for 5 minutes. Cut into ½ inch thick slices against the grain of the meat.