Cut each pork tenderloin crosswise into 6 pieces. Press each piece, cut side down, to form a round, 1 to 1 1/4 inches thick.
Place rosemary and thyme sprigs in shallow glass or plastic dish or resealable plastic food-storage bag. Stir in vinegar and oil. Add pork; turn to coat. Cover dish or seal bag and refrigerate, turning pork 2 or 3 times, at least 1 hour but no longer than 24 hours.
Heat coals or gas grill for direct heat. Remove pork from marinade; reserve marinade.
Cover and grill pork over medium heat 7 minutes, turning and brushing frequently with marinade. Discard any remaining marinade. Cover and grill pork 6 to 8 minutes longer, turning frequently, until pork has slight blush of pink in center and meat thermometer inserted in center reads 160º.