Pour half the marinade into each bag, press out air and seal. Refrigerate for 12 to 24 hours. Turn over two or three times while marinating. Cover and cook on Low for 7 to 8 hours. About halfway through cooking time, carefully reverse ribs so that they all cook part of the time in the liquid.
Meat is done when it is tender and falling off the bone.
Makes 6 to 8 servings. Cook for 9 hours on High, uncovered. Remove vanilla bean and whisk milk mixture gently. Use a dull-edged knife to carefully scrape down the crust of sugar that accumulates on sides of the insert. Do not skim foam.