1. In a large bowl, mix vinegar, oil, shallots, oregano, and molasses.
2. Peel onions. Trim off and discard stem ends of pattypan squash and zucchini. Cut onions and pattypan into about 1 1/2-inch-wide wedges. Cut zucchini into 1 1/2-inch chunks.
3. Trim stems and seeds from peppers, then cut peppers into 1 1/2-inch squares.
4. Put all the vegetables in bowl with vinegar mixture and stir to coat. Cover and let stand at least 10 minutes or up to 2 hours, stirring occasionally.
5. Lift vegetables from marinade and place in a wire grilling basket or thread vegetables onto thin metal skewers; leave marinade in bowl.
6. Lay vegetables on a barbecue grill over a solid bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds). Close lid on gas grill. Turn vegetables as needed until browned, 8 to 10 minutes.
7. Put vegetables (push off skewers) back into marinade. Mix and serve vegetables warm or tepid; they can stand up to 2 hours. Add salt and pepper to taste.
A wire grilling basket makes the vegetables easier to manage.