The day before you plan to serve, pour 4 cups of the root beer into ice cube trays to a depth of no more than 1/2-inch. Keep the remaining root beer refrigerated until ready to serve. Freeze overnight, along with tall, thin glasses or parfait glasses for serving. When ready to serve, unmold the cubes into a food processor fitted with the metal blade. Process, pulsing, just until crushed. Place a straw in each glass, and layer the parfaits by alternating scoopfuls of the root beer granita and vanilla ice cream, ending with a scoop of ice cream on the top. Serve with a pitcher of the remaining root beer, and pour root beer up to the top of each parfait at the table.