1. Place chicken in slow cooker. Sprinkle with onion and garlic. Whisk together barbecue sauce, vinegar and Tabasco; pour on top. Cover and cook on low until chicken is cooked through, 5 to 6 hours. Remove chicken from slow cooker and let cool slightly; shred. Stir chicken back into sauce.
2. Arrange chicken on buns or lettuce leaves, if desired. Top with coleslaw and tops of buns, then serve immediately.