Score ham about ½-inch deep with a sharp knife. Place ham on a rack in a shallow baking pan. Bake at 325 degrees for 1¾ to 2¼ hours or until a thermometer reads 140 degrees.
In a small bowl, combine the brown sugar, mustard and enough vinegar to make a thick paste. During the last hour of baking, brush glaze on ham every 15 minutes.