Mix the paprika, garlic powder, onion powder, salt, pepper, and cumin. Rub all over the pork.
Scatter the sliced onion in the bottom of a 6-quart slow cooker. Set the pork on top.
Pour the broth and vinegar around (not over) the pork so the rub stays put.
Cover and cook on low for 8 to 10 hours or high for 4 to 5 hours, until the pork shreds easily with a fork.
Lift the pork out, shred it with two forks, and discard any large pieces of fat.
Skim fat from the cooking liquid, then stir the shredded pork back in with the BBQ sauce. Cook 15 more minutes on low so the sauce soaks in.
Pile onto toasted buns and top with coleslaw.