To butterfly the chicken: Get a sharp paring knife and a pair of kitchen shears. Place the chicken on a cutting board breast side down. Working from the cavity opening up to the neck, cut along each side of the backbone with the kitchen shears. Discard the backbone or reserve it for stock. Grab the chicken with one breast in each hand and open it like a book, exposing the cavity of the bird—you’ll hear a crack, most likely. Running from the middle of the bird down between the two legs is the so-called “soft bone” or “keel bone”—this is the breastbone. It’s a triangular shape that is firm toward the middle of the bird but becomes soft flexible cartilage as it tapers down between the legs. To remove it, use a paring knife to cut down either side of it. Once you’ve cut it free, use your fingers to loosen and then pull out the breastbone. Discard the bone, and your chicken is good to go. Rub the chicken all over with the vinegar, then season it on both sides with salt and pepper. Generously smear the rub under and over the skin and on the interior of the bird. Place in a baking pan, cover with plastic wrap, and refrigerate at least three hours or preferably overnight.