Preheat oven to 350 degrees F. Place tart shells on a baking sheet, and then bake 10 minutes. Remove from oven and allow tart shells to cool. While this occurs, beat Mascarpone cheese with the sugar until light. Beat in eggs and vanilla. Divide Mascarpone mixture among the tart shells. Bake 20 minutes, or until filling is puffed and set. Cool tarts to room temperature, and then top each with 3 green, and 3 red grapes. Tarts can be made to this stage several hours in advance and kept stored in the fridge. Dust with icing sugar just before serving. Makes 12 tarts.