Scrub ham thoroughly with a stiff brush. Place ham in a very large container; cover with water. Add vinegar and 1 cup brown sugar. Cover and bring to a boil. Reduce heat, and cook 3 hours and 20 minutes.
Allow ham to cool in cooking liquid. Remove ham from liquid; remove skin. Place ham, fat side up, on a cutting board; score fat in a diamond design.
Place ham in a shallow roasting pan. Combine dark brown sugar, cornmeal, and mustard; pat over surface of ham, coating well. Bake at 350° for 20 minutes or until glaze is melted. Cover and chill in refrigerator overnight. Cut into paper thin slices. Serve with beaten biscuits.
Note: Leftover ham may be refrigerated for later use.