In a medium bowl, whisk together the vinegar, sugar, ketchup, soy sauce and water. Set aside.
Place cornstarch in a ziplock bag. Add chicken and shake to coat evenly.
In a small bowl, add the beaten egg. In another bowl, add the flour. Set aside along with a wire cooling rack.
Place a large skillet over medium-high heat. Add oil and heat to 350˚F until almost smoking.
Dip 6-8 pieces of chicken into the egg, and then the flour. Shake off excess and add to the oil.
Fry 2-3 minutes until golden and crispy, and remove to the rack with a slotted spoon. Repeat for the remaining chicken pieces.
Remove and drain oil. Set aside.
Add onion, bell peppers and pineapple to the pan. Sauté for about 2 minutes until slightly softened.
Add the reserved sauce mixture and bring to boil.
Add in the cooked chicken pieces and stir to coat evenly with the sauce. Serve and enjoy!