Roll out pastry for bottom crust, and place in a pie dish. Spoon the rhubarb into the crust. In a large bowl, beat the eggs slightly. Mix in sugar, flour, vanilla, milk, and butter or margarine. Pour mixture over rhubarb. Cover with top crust, and seal the edges. Bake at 400 F for 50 to 60 minutes or until the custard is set and a toothpick when inserted comes out clean. Serves: 6 to 8.