Trim fat from meat. If necessary, cut meat to fit into a 3 1/2- or 4-quart slow cooker. With a sharp knife, make slits evenly on all sides of the meat. Insert garlic slices into slits. Place meat in cooker.
In a blender combine vinegar, cilantro, onion wedges, the water, oregano, cumin, salt, and pepper. Cover and blend until smooth. Pour over meat in cooker.
Cover and cook on low-heat setting for 10 to 12 hours or on high-heat setting for 5 to 6 hours.
Just before serving, in a large skillet cook red onions in hot oil over medium heat about 15 minutes or until tender. Carefully add lime juice to skillet. Cook and stir for 3 to 5 minutes more or until lime juice is evaporated.
Meanwhile, using a slotted spoon, transfer meat from slow cooker to a cutting board. Using two forks, pull meat apart into shreds. Place shredded meat in a large bowl. Add 1 cup of the cooking juices in cooker, tossing to coat.
Divide shredded meat among bottoms of buns. If desired, drizzle with additional cooking liquid. Top with cooked red onions and bun tops.