Pour the grape juice into a medium saucepan. Add the sugar and stir to mix. Bring to a boil over high heat, then reduce the heat to medium and cook for 6 to 8 minutes, or until reduced to 2 cups. Stir in the vanilla. Pour into a 9'' or 10'' freezerproof glass pie plate and set aside to cool.
Place in the freezer for 1 to 1½ hours, or until the edges are mostly solid and the middle is still liquid. Mash with a fork to break up any large ice crystals.
Return to the freezer for 1 hour, or until frozen through but not hard. Break up the crystals with a fork before serving.
Note : After the second freezing the slush can be stored in a covered container in the freezer for up to 1 day before serving. Remove from the freezer and let stand at room temperature for 5 to 10 minutes before serving.