Place the pork roast in the crock pot. Add the chopped onions around the roast. In a bowl, combine the barbecue sauce, vinegar, broth, brown sugar, mustard, Worcestershire sauce, chili powder, garlic, and thyme. Mix well. Pour over the roast, covering it completely in the sauce. Cover the crock pot and cook on high heat for 5 hours or until the meat shreds easily with a fork. Remove the pork from the crock pot and place on a cutting board or platter. Shred the meat using two forks. Return the shredded meat to the crock pot and stir into the sauce. Let the pork sit in the sauce (with the crock pot still on high heat) while you prepare the buns. Heat a skillet over medium heat. Butter the insides of the hamburger buns. Place the buns butter side down in the skillet and heat until toasted. Serve the shredded pork on the toasted buns.