Combine brown sugar, paprika, salt and pepper rub over roast. Place roast over onions.
Combine vinegar, Worcestershire sauce, red pepper flakes, sugar, mustard, garlic salt and cayenne stir to mix well. Drizzle all of vinegar mixture over roast.
Cover and cook on Low for 10 to 12 hours or on High 5 to 6 hours. Remove meat and onions leaving 1/3 of the drippings in the crock pot. Chop or shred meat and chop onions and place the meat back
into the crockpot with the remaing drippings on low for 30 minutes.
Serve meat and onions on buns, along with Sweet Baby Rays BBQ Sauce. If desired, top sandwiches with coleslaw.