Season chicken breasts with salt and pepper on all sides. Place in a slow cooker; pour 1/4 cup barbecue sauce and vinegar on top.

Cook on Low for 3 hours.

Combine brown sugar, onion powder, garlic powder, and red pepper flakes in a small bowl.

Pour out liquid from the slow cooker and add remaining barbecue sauce. Sprinkle chicken with brown sugar-spice mixture.

Cook on Low, stirring occasionally, until chicken is no longer pink in the center and the juices run clear, about 3 more hours. An instant-read thermometer inserted into the center should read at least 165 degrees F (74 degrees C).