In a bowl, toss chicken with 4 tablespoons dressing. Brush zucchini and onion with remaining 2 tablespoons dressing., Place corn, zucchini and onion on a grill rack over medium heat; close lid. Grill zucchini and onion 2-3 minutes on each side or until tender. Grill corn 10-12 minutes or until tender, turning occasionally. , Drain chicken, discarding marinade. Grill chicken, covered, over medium heat 3-4 minutes on each side or until no longer pink., Cut corn from cobs; cut zucchini, onion and chicken into bite-sized pieces. In a 3-qt. trifle bowl or other glass bowl, layer romaine, cucumber, grilled vegetables and chicken. If desired, serve with additional dressing.