Heat the oven to 300°F and remove the papery membrane from the bone side of each rib rack.
Mix the brown sugar, paprika, salt, pepper and garlic powder, then rub it evenly over both sides of the ribs.
Place each rack meat side up on a foil-lined rimmed pan and cover the pans tightly with foil.
Bake for 2 1/2 hours until the meat has pulled back from the bones and the thickest section is about 195°F to 203°F, well above the 145°F minimum safe temperature for pork.
Stir the barbecue sauce and vinegar together, uncover the ribs and brush them with half the sauce.
Increase the oven to 450°F and bake uncovered for 10 minutes, brush again, then bake 5 to 8 minutes until sticky with darkened edges.
Rest the racks for 10 minutes before cutting between the bones.