Mix first 5 ingredients and 1 Tbsp. vinegar until blended.
Cut meat into 12 pieces. Place in slow cooker; cover with half the barbecue sauce mixture. Top with onions. Cover with lid. Cook on LOW 6 to 8 hours (or on HIGH 3 to 4 hours).
Transfer meat and onions to large bowl; discard cooking liquid from slow cooker. Pour remaining barbecue sauce mixture into microwaveable bowl; cover with waxed paper. Microwave on HIGH 1 min. or until warmed. Add to meat mixture; toss to evenly coat.
Combine broccoli slaw, mayo and remaining vinegar. Fill buns with meat mixture, Singles pieces and slaw just before serving.