1. Whisk together pepper jelly, red pepper, 1/3 cup red wine vinegar, and 1/2 tsp. each salt and pepper in a shallow dish or zip-top plastic freezer bag. Add chicken, turning to coat. Cover or seal, and chill 30 minutes, turning occasionally.
2. Preheat grill to 350° to 400° (medium-high) heat. Thread onion pieces and next 2 ingredients alternately onto 5 skewers, leaving a 1/8-inch space between pieces. Place skewers in a shallow pan. Whisk together olive oil and remaining 1/3 cup vinegar, 1/2 tsp. salt, and 1/2 tsp. pepper. Pour over vegetables.
3. Remove chicken from marinade, discarding marinade. Thread chicken onto remaining 3 skewers, leaving a 1/8-inch space between pieces.
4. Grill kabobs, covered with grill lid, 6 to 8 minutes on each side or until chicken is done and vegetables are crisp-tender.