In a small bowl, whisk together ketchup, rice vinegar, brown sugar, soy sauce, and 1/2 cup water to make the sauce.
In a large skillet or wok, heat vegetable oil over medium-high heat. Add the pork and cook until browned and cooked through, about 5-7 minutes. Remove pork from skillet and set aside.
In the same skillet, add the garlic, onion, bell peppers, and pineapple. Cook until vegetables are tender, about 5 minutes.
Return the pork to the skillet and pour in the sauce. Stir to coat the pork and vegetables. Bring to a simmer and cook for 2-3 minutes, until the sauce has thickened.
Season with salt and pepper to taste. Serve hot with steamed rice.