Heat gas or charcoal grill. Brush chicken with 1 tablespoon of the dressing. Carefully brush oil on grill rack. Place chicken, zucchini and onion on grill over medium-high heat. Cover grill; cook 6 to 8 minutes, turning once, or until chicken is no longer pink in center and vegetables are tender. Add tomato halves to grill for last 4 minutes of cooking time.