Make the crust by lightly coating a 9 by 9 inch or 7 1/4 inch by 11 1/4 inch baking pan with nonstick vegetable spray.
In a mixing bowl or food processor, mix together the flour, sugar and butter until it just forms into a dough. Lightly pat it over the bottom of the pan, making sure that it clings 1/4 inch up the sides. With a form, prick little holes in the dough. Refrigerate at least 1 hour, until firm.
Preheat oven to 375 degrees F. Bake the crust for 15 minutes on a rack in the middle of the oven. It is done when lightly tanned and set.
While the crust is baking, make the filling by combining the sugar and flour in a medium bowl. Beat in the eggs, then the lemon zest, juice and vanilla.
Pour the filling over the finished crust and bake 15 minutes, until set. Set the pan on a rack to cool. When it is cold, lightly dust with confectioners' sugar. (If you do it when it isn't completely cold, the sugar will just melt and disappear.) Cut into 2 inch pieces.Makes 12 bars.