Step 1Prepare the ribs: Peel the membrane from the back of each rack of ribs. Rinse the ribs under cold water and pat dry with paper towels.
Step 2Season the ribs with salt, using ½ teaspoon kosher salt for each pound of ribs. After rubbing the salt evenly over the ribs, season each rack with rub, rubbing about 2 tablespoons evenly over each rack. Refrigerate the racks until ready to smoke, up to several hours.
Step 3Meanwhile, prepare your smoker or grill to cook over low, indirect heat. Set up a drip pan underneath where the ribs will smoke, and fill with water. Shortly before cooking, adjust the heat as needed to maintain a temperature around 250 degrees, and add hickory chips to start smoking. Meanwhile, prepare your baste: In a bowl, whisk together the vinegar, water and mustard.
Step 4Place a rib rack over the drip pan and slide the ribs onto the racks. Adjust the heat as needed (add several coals to either side of the grill as needed if using a kettle grill) to maintain the ambient temperature (about 250 degrees). Replenish the chips as needed to keep smoking. Baste the ribs every 30 minutes or so to keep it moist.
Step 5After about 1 1/2 hours, baste the ribs once more. Wrap the ribs tightly in foil and continue to cook over indirect heat (about 250 degrees) until the meat is just tender (a toothpick slid into the meat between the bones should penetrate easily, and the meat will crack as the rack is bent), 2 to 3 additional hours (time may vary depending on the size of the smoker and how well the heat is regulated). The ribs can also be finished in the oven: Place the basted ribs in a roasting pan tightly covered with foil and cook in a 250-degree oven until tender, 2 to 3 hours.
Step 6Uncover the racks and and lightly coat them with barbecue sauce. Place the ribs directly over the coals or a hot grill and cook until the sauce is aromatic and begins to darken, about 5 minutes. Watch the ribs to make sure the sauce does not burn.