Place mayonnaise and bell pepper in a blender or food processor, and process until smooth.
Brush oil over zucchini, eggplant, and onion; sprinkle with salt and black pepper. Heat a large grill pan coated with cooking spray over medium-high heat. Add zucchini, and cook for 3 minutes on each side or until tender. Place in a large bowl; keep warm. Add eggplant to pan, and cook for 3 minutes on each side or until done. Add to zucchini; keep warm. Add onion to pan; cook for 3 minutes on each side or until done. Add to eggplant mixture; drizzle with vinegar.
Lightly coat the bread with cooking spray. Place 2 bread slices in pan; cook 1 minute on each side or until toasted. Repeat procedure with remaining bread. Rub 1 garlic half on 1 side of each bread slice. Spread 1 tablespoon mayonnaise mixture over the garlic-rubbed sides of 4 bread slices; layer each with 3 slices zucchini, 2 slices eggplant, and 1 slice onion. Spread 1 tablespoon mayonnaise mixture over 1 side of each remaining bread slice; place on top of sandwiches.