Bring 6 cups salted water to a boil; add shrimp, and cook 3 minutes or just until shrimp turn pink. Drain and rinse with cold water. Peel shrimp, leaving tails on. Devein, if desired.
Combine vinegar and next 6 ingredients in a shallow dish; add shrimp alternately with onion. Cover and chill 6 to 24 hours, turning often. Drain shrimp mixture, discarding liquid, before serving.