Source: Moosewood Restaurant Low-Fat Favorites by Molly Katzen
Combine the juice and sugar in a saucepan. Pour the dissolved potato starch mixture into the saucepan and slowly bring it to a simmer on medium-high heat, about 10 minutes. As soon as it begins to simmer, stir for about 5 minutes, until the cloudy juice becomes clear. Stir in the strawberries and raspberries and remove from the heat. Stir in the vanilla. Refrigerate at least 20 minutes.