In bowl, toss chicken with 1/3 cup of the stir-fry sauce; let stand 30 minutes. Combine remaining 1 2/3 cups stir-fry sauce, vinegar and red pepper; set aside. Dissolve cornstarch in 3/4 cup water; set aside.
In rondo or large sauté pan, heat 1/3 cup of the oil over high heat. Add chicken and garlic and stir-fry 3 minutes; remove. Heat remaining 2/3 cup oil in the same pan. Add onions; stir-fry 1 minute.
Add bell peppers; stir-fry 4minutes. Add chicken, stir-fry sauce mixture, cornstarch mixture and peanuts. Cook and stir until sauce boils and thickens.