In a medium-size bowl, stir together sour cream, mayonnaise, scallions, vinegar, seasoning blend, salt and black pepper. Cover and refrigerate until ready to serve.
Heat gas grill to medium-high or coals in charcoal grill to medium hot. Lightly coat grill rack with oil or cooking spray.
Brush asparagus, peppers, squash and fish with oil. Sprinkle all sides with the seasoning blend. Grill vegetables about 8 to 10 minutes, turning as needed so vegetables do not burn. Grill fish flesh-side down 5 minutes. Turn and grill additional 3 minutes or until cooked through.
Sprinkle fish and vegetables with 1/8 teaspoon salt and serve with sauce.