Coat a large skillet with cooking spray; heat over medium-high heat. Cook chicken in hot skillet until brown. Drain off any fat. In a 3-1/2- or 4-quart slow cooker combine chicken, pizza sauce, vegetables, and tomatoes.
Cover and cook on low-heat setting for 6 to 8 hours or on high-heat setting for 3 to 4 hours.
Preheat broiler. Arrange buns, cut sides up, on the rack of an unheated broiler pan. Broil 3 to 4 inches from the heat for 1 to 2 minutes or until toasted.
Spoon chicken mixture onto bottoms of buns. Add cheese and tops of buns.