Cut zucchini lengthwise into 1/4-inch-thick oblong slices.Cut each bell pepper into 6 wedges, and cut onion into 1/2-inch-thick slices.
Combine vinaigrette and molasses in a large heavy-duty zip-top plastic bag. Add vegetables; seal bag, and chill 2 hours, turning bag occasionally.
Remove vegetables from bag, reserving marinade; place in a lightly greased grill basket.
Cutbread loaf in half horizontally, and brush cut sides with 3 tablespoons reserved marinade.
Grill vegetables, without grill lid, over medium-high heat (350 to 400 degrees) 5 minutes, basting occasionally with remaining marinade. Turn basket over, and grill, basting occasionally, 2 minutes.
Place bread, cut-sides down, on grill rack; grill 3 minutes or until vegetables are tender and bread is toasted.
Combine crumbled feta cheese and mayonnaise; spread over cut sides of toasted bread. Place grilled vegetables on bottom half of bread. Sprinkle with Parmesan cheese, and top with remaining bread half. Cut into 8 pieces.