Soak gelatin in the 1/2 cup water. Scald 2 cups of the milk and dissolve the gelatin in the hot milk. Add the sugar and stir until dissolved. Cool to room temperature and add the evaporated milk, the 2 1/2 quarts of milk and the vanilla. Chill and freeze in a 6 quart freezer.
OPTIONAL: A pint of half & half may be substituted for 2 cups of the milk.