Place mix rice vinegar, soy sauce, sugar, and cornstarch in a large, zippered bag. Add chicken and toss to coat. Set aside.
In a small bowl, whisk soy sauce, brown sugar, balsamic vinegar, chili powder, water, ginger, and cornstarch. Set aside.
Heat ½ tablespoon of oil in a skillet and swirl it around. Add peanuts and red pepper flakes and cook them until they are lightly browned. Transfer these to a bowl.
Add remaining oil to the skillet and saute the peppers, broccoli, and garlic for about a minute. Add to the bowl with peanuts.
Remove chicken from marinade (discard leftover marinade) and fry in the skillet until cooked through (about 2 minutes). Add green onions.
Return the peanuts and vegetable mixture to the skillet with the sauce and simmer until the sauce is thickened, about 1 minute.
Serve Kung Pao Chicken over a bed of steaming rice if desired.