Arrange the onion over the bottom of a 6-quart slow cooker. Mix the brown sugar, paprika, salt, celery salt, chili powder and 1 tsp. pepper in a bowl. Rub over the surface of the pork and place it in the cooker. (I put the meat in with the fat on top. I do not trim the fat until AFTER I cook the meat, but some people don't trim it at all. (they mix it in for flavor)
Mix the remaining ingredients (NOT BUNS! lol) and pour half over the pork. Reserve remaining half. Cook on High for one hour. Reduce heat to low and cook for 7 hours longer. Shred the pork and serve with the remaining vinegar sauce.
Serve on buns, add choice of slaw and different barbecue sauces.