In a bowl, toss the shrimp with cornstarch until evenly coated.
Heat the vegetable oil in a large skillet over medium-high heat. Add the shrimp and cook for about 2-3 minutes until they turn pink. Remove the shrimp from the skillet and set aside.
In the same skillet, add the bell pepper, onion, and garlic. Sauté for about 3-4 minutes until the vegetables are tender.
In a separate bowl, mix together the ketchup, rice vinegar, brown sugar, soy sauce, and sesame oil. Pour this mixture into the skillet with the vegetables and bring to a simmer.
Add the cooked shrimp and pineapple chunks to the skillet. Stir to combine and cook for an additional 2-3 minutes until everything is heated through. Season with salt and pepper to taste.
Serve the sweet and sour shrimp over cooked rice.