In a resealable plastic bag, combine vinegar, sugar, garlic, rosemary, 1 teaspoon salt, and 1/2 teaspoon pepper. Add meat to marinade, and turn to coat. Let marinate at room temperature at least 15 minutes, or cover and refrigerate up to 1 day.

In a grill pan over medium-high heat, add oil. Remove steaks from marinade, allowing excess to drip off. Grill steaks 2 to 3 minutes then flip and cook another 2-4 minutes for medium-rare. Transfer to a plate; cover loosely with aluminum foil, and let rest 5 minutes.