In a large mixing bowl, season the chicken pieces with salt and pepper. Add the flour and toss until the chicken is evenly coated.
Heat the vegetable oil in a wok or large skillet over medium-high heat. Once hot, add the coated chicken pieces in batches. Fry until golden brown and cooked through, about 4-5 minutes per batch. Use a slotted spoon to transfer the fried chicken to a paper towel-lined plate to drain excess oil.
In the same wok, add the diced bell peppers and onion. Stir-fry for about 2-3 minutes until they begin to soften.
Add the pineapple chunks to the vegetables and stir for another minute.
In a separate bowl, whisk together the soy sauce, rice vinegar, brown sugar, and ketchup. Pour this mixture into the wok and bring to a simmer.
Once the sauce is bubbling, add the cooked chicken back into the wok. Stir well to combine everything.
Add the cornstarch mixture to the sauce, stirring continuously until the sauce thickens, about 1-2 minutes. Adjust seasoning with salt and pepper as needed.
Serve the Sweet and Sour Chicken hot over cooked rice.