Whisk together the water, rice vinegar, brown sugar, ketchup, soy sauce, and cornstarch in a medium bowl. Set aside.
Place a large skillet over medium-high heat. Add 1 tablespoon of oil to the pan. Once hot, cook the chicken pieces until cooked through, about 10 minutes. Transfer the chicken to a bowl and set aside.
To the same skillet, add the other tablespoon of oil. Then add the onion and bell peppers. Cook until the veggies are softened, about 8-10 minutes
Add the pineapple, cooked chicken, and sauce to the skillet. Cook until the sauce thickens to your desired consistency.