Place chicken into bottom of crock. In a mixing bowl, whisk together balsamic vinegar, maple syrup, garlic, salt, and chili flakes. Pour over chicken. Add onion, sliced peppers, and zucchini to slow cooker.
Cover and cook on low 4-5 hours or on high 3 hours.
Mix cornstarch and water together to form a smooth paste. Add to slow cooker, and mix into the cooking liquid. Turn crockpot to high, and cook another 30 minutes until mixture thickens.
Cook rice according to package directions. Serve teriyaki chicken over rice, and garnish with chopped cilantro, if desired.