Make the marinade: Combine olive oil, vinegar, oregano, chili powder, sugar, garlic salt, salt, and pepper in a large bowl.
Prepare fajitas: Add zucchini, yellow squash, onion, and bell peppers to the marinade. Marinate vegetables in the refrigerator for at least 30 minutes, but not more than 24 hours.
Heat oil in a large skillet over medium-high heat. Drain vegetables and sauté until tender, about 10 to 15 minutes. Stir in corn and black beans. Increase the heat to high to brown vegetables, about 5 minutes.
