Place the roast, onion, bay leaves, black pepper, garlic powder and vinegar in 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 slow cooker and discard liquids. Return meat to slow cooker; shred with fork and knife. Stir tomato pure, chilli powder and salt into the shredded meat. Cover the cooker then cook meat and sauce on High for an additional 2 hours.
This flavourful beef is great with tortillas and cheese or used in your favourite taco or enchiladas.