Wash and pick over the blueberries and clean, hull and slice the strawberries (or otherwise prepare your fruit - you can use your imagination here!) Mix the sugar with 4 cups of the juice and bring to a simmer. Mix the corn starch with the remaining 1/2 cup of juice. Pour into the simmering pot and stir constantly until thickened. Add the vanilla. Mix in the berries and immediately remove from the heat. Let cool and refrigerate until ready to serve.