Place roast, onion, bay leaves, black pepper, garlic powder, and vinegar in a slow cooker; cover ingredients completely with water. Cover and cook on High setting until meat is very tender and falling apart, about 4 hours.

Remove meat from the slow cooker and discard liquids. Return meat to slow cooker; shred with fork and knife. Stir tomato sauce, chili powder, and salt into the shredded meat. Cover the cooker, and cook meat and sauce on High for an additional 2 hours.