Combine bell pepper, onion, and pineapple in the basin of a slow cooker. Place chicken breasts on top.In a small bowl, whisk together the vinegar, sugar, ketchup, soy sauce, ginger, and garlic. Pour over the chicken. Cover and cook on high for 3-4 hours.Remove chicken to a cutting board and cube. Remove 1 cup of hot liquid from the slow cooker to a small bowl. Whisk in the cornstarch until no lumps remain. Pour slurry into the slow cooker and stir until smooth and sauce has thickened. Return the chicken to the slow cooker and toss to coat in sauce. Serve over rice.