Preheat oven to 325°. Place ham on a rack in a shallow roasting pan. Using a sharp knife, score surface of ham with 1/4-in.-deep cuts in a diamond pattern. Cover and bake until a thermometer reads 130°, 1-1/2 to 2 hours., Meanwhile, in a small bowl, combine the brown sugar, mustard and enough vinegar to make a thick paste. Remove ham from oven. Spread sugar mixture over ham. Bake, uncovered, until a thermometer reads 140°, 15-30 minutes longer.