Trim fat from chops. Place chops in a plastic bag set in a shallow dish. For marinade, in a small saucepan combine cider vinegar and balsamic vinegar. Bring to boiling; reduce heat. Boil gently, uncovered, about 8 minutes or until reduced to about 2/3 cup. Cool slightly. Stir in oil, sage, salt, and pepper. Reserve 1/4 cup marinade; set aside.
Pour remaining marinade over chops; seal bag. Marinate in the refrigerator for 8 to 24 hours, turning bag occasionally. Drain chops, discarding marinade. Brush potatoes and onion with the reserved marinade.
For a charcoal grill, place medium coals in bottom of grill. Place chops, potatoes, and onion on grill rack directly over coals. Cover and grill for 25 to 30 minutes or until juices run clear and vegetables are tender, turning chops once and vegetables occasionally. (For a gas grill, preheat grill. Reduce heat to medium. Place chops, potatoes, and onion on grill rack over heat. Grill as above.) Makes 4 servings.