Place shoulder in slow cooker. Add enough water/vinegar mixture (2 parts vinegar, 1 part water) to completely cover. Depending on the size of the cooker and the size of the pork, you may require more or less liquid than the ingredients suggest. Add salt, allowing the salt to accumulate on top of the meat. Refrigerate brine solution 12-24 hours.
Remove some liquid from brine solution until about 1/2" of pork is exposed. Cut onion into eights and add to the mixture. Spread spices on exposed pork surface evenly, then cover with heaps of brown sugar.
Cook in slow-cooker 8-10 hours.
After cooking, remove pork from slow-cooker (be careful: it will probably be falling apart). Scrape off external fat. Shred (with a pair of forks).
Serve in sandwich buns, with your favorite barbecue sauce if so inclined.