Cut squash and zucchini in half crosswise. Cut halves lengthwise into 1/2-inch-thick wedges. Cut bell peppers into 2-inch-long strips. Set pepper strips aside.
Coat a large baking sheet with cooking spray. Place squash and zucchini in a single layer on baking sheet; coat vegetables with cooking spray. Sprinkle with oregano, salt, and pepper.
Bake at 500° for 5 minutes. Turn squash and zucchini gently; add pepper strips. Bake an additional 8 minutes or to desired degree of doneness. Transfer vegetables to a serving bowl. Add vinegar to vegetables, and toss gently.