Prepare grill or broiler.
Sprinkle steak with 1/4 teaspoon salt and 1/4 teaspoon black pepper.
Combine 1/2 teaspoon of salt, 1/4 teaspoon of black pepper, vinegar, and next 5 ingredients (through garlic) in a large zip-top plastic bag. Seal and shake to coat.
Place steak on grill rack or broiler pan coated with cooking spray; cook 4 minutes on each side or until desired degree of doneness. Let stand 5 minutes; cut steak into 1/4-inch slices. Place zucchini and squash on grill rack or broiler pan coated with cooking spray; cook for 4 minutes on each side or until tender. Place bell peppers and onions on grill rack or broiler pan; cook 2 minutes or just until tender.
Coarsely chop vegetables; place in a bowl. Add pesto; stir gently. Garnish with oregano, if desired.
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