Toss zucchini, squash, eggplant, onion, corn, and peppers with ⅛ cup olive oil in a large bowl. Grill over medium-heat until tender and lightly charred, about 5 to 7 minutes for the zucchini, squash, and eggplant, and about 8 to 10 minutes for the peppers, onion, and corn. Remove from heat.