1. Place garlic on a flat surface. Press each head firmly to separate cloves. Discard root end and loose peels; do not peel cloves. Thread cloves, touching, onto thin metal skewers.
2. Lay skewers on a barbecue grill over a solid bed of very hot coals or high heat on a gas grill (you can hold hand over grill only 2 to 3 seconds); close lid on gas grill. Turn as needed until garlic is soft when pressed and peels are charred, about 10 minutes.
3. When cool enough to touch, in 3 to 4 minutes, push garlic off skewers and squeeze cloves from peels; discard peels.
4. In a blender or food processor, whirl garlic, butter, wine, broth, and vinegar until pureed. Scrape into a 5- to 6-quart pan. Add clams; cover pan.
5. Bring to a boil over high heat (see notes) and cook until clams open, about 10 minutes. Ladle clams and broth into wide bowls. Serve with chunks of bread to dip into sauce.