1. Bring 7 1/2 cups water to a boil; add shrimp, and cook 2 to 3 minutes or until shrimp turn pink. Drain and rinse with cold water. Peel shrimp, and, if desired, devein.
2. Layer shrimp, red onion slices, and bell pepper slices in an airtight container.
3. Whisk together vegetable oil and next 9 ingredients; pour over shrimp. Cover and chill 24 hours, stirring occasionally.
4. Stir in 1/2 cup chopped basil 1 hour before serving.