Mix 1 Tbsp of the brown sugar, the paprika, minced onion, chili powder, salt, garlic powder and allspice. Sprinkle 1 Tbsp into bottom of slow-cooker. Rub remaining mixture over pork. Add vinegar and remaining brown sugar to slow-cooker; stir to mix. Add pork.
Cover and cook on low 8 to 10 hours until pork is fork-tender. Remove pork to cutting board and pull into shreds. Return to slow-cooker; add scallions and toss to coat.
Put about 1/4 cup onto each bun bottom. Top with bun tops.