In the bowl of a food processor combine the flour, sugar, and salt; process to blend. Add the cold butter and pulse in six 1-second pulses; the mixture should look sandy. In a cup with a fork, mix the egg yolk, water, and vanilla; drizzle over the flour mixture and then pulse 5 times to incorporate (mixture should still be crumbly looking.) Transfer the mixture to a 9 ½-inch metal tart pan with a removeable bottom. Pat well, with lightly floured hands, into the bottom of the pan, until you have a smooth pastry base. Chill for 15 minutes while you heat the oven to 325 F.

Bake the pastry for 25 to 30 minutes, until slightly springy when pressed with a finger, lightly golden, and just beginning to brown on the edge. Let cool.

In a medium bowl, whisk together the cream cheese, sour cream, and 2 tablespoons sugar. Whisk in the lemon juice. Spread the cream cheese mixture over the tart base, leaving a 1/2-inch border. Arrange the grapes in concentric circles over the cream cheese mixture. Chill until ready to serve.