Combine the vinegar, soy sauce, garlic, ginger, orange peel and orange juice in a nonmetallic bowl; add shrimp and scallops and toss them gently to coat seafood. Cover and refrigerate 30 minutes, stirring occasionally.
Cook pasta according to package directions. While pasta is cooking, heat oil in a nonstick wok or skillet; add shrimp mixture (do not drain). Add hot red pepper flakes, if desired. Stir-fry 2 minutes. Add snow peas, carrots and bell pepper; cover and cook 2 minutes or until shrimp are pink and scallops are opaque. Dissolve cornstarch in water; stir into wok. Cook 1 minute. When pasta is done, drain well. Transfer to serving bowl and pour stir-fry over top. Toss lightly.