Coat chicken pieces with cornstarch evenly.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add chicken and cook until golden brown and cooked through, about 6-7 minutes.
Add the sliced peppers and onions, and stir-fry for 3-4 minutes until slightly softened.
In a small bowl, whisk together ketchup, rice vinegar, sugar, and soy sauce.
Pour the sauce into the pan with the chicken and vegetables, stir well.
Add the cornstarch slurry and stir until the sauce thickens, about 2 minutes.
Serve hot over rice or noodles if desired.