Cut bell peppers into large pieces, discarding seeds and membranes. Cut zucchini in half crosswise; cut each piece in half lengthwise. Cut onions into large pieces. Slice eggplant into 1/2-inch slices. Add vegetables to marinade; toss to coat. Cover and chill 2 hours.
Remove vegetables from marinade, reserving marinade. Grill, without grill lid, over medium-high heat (350° to 400°) 10 to 12 minutes or until just tender, basting occasionally with reserved marinade. Serve warm or at room temperature.