Preheat oven to 350 degrees F. Place the tart shells on a baking sheet. Prick each a few times with a fork (this will prevent puffing when they bake). Bake the tarts 10 minutes. Remove from oven and cool 10 minutes.
While that occurs, place the cream cheese, sugar and vanilla in a bowl. Beat with an electric mixer until well combined. Add the egg and beat until the mixture is very smooth. Divide and spoon the cream cheese mixture into the tart shells. Bake 20 minutes, or until filling is set. Cool tarts to room temperature, and then refrigerate until ready to serve.
When ready to serve, remove the foil liner from each tart. Top each tart with 6 grapes, dust with icing sugar, and enjoy.