PLACE chicken in shallow dish. Mix herbs, garlic and oil until blended; pour over chicken. Turn chicken to evenly coat both sides of each breast. Refrigerate 30 min. to marinate.
HEAT grill to medium heat. Remove chicken from marinade; discard marinade. Grill chicken 5 min.; turn. Add vegetables; grill 5 min. or until chicken is done (165ºF) and vegetables are crisp-tender, turning vegetables occasionally.
FILL bread slices with chicken, vegetables and Singles to make 4 sandwiches. Spread outsides of sandwiches with mayo. Grill 3 to 5 min. on each side or until golden brown on both sides.