Preheat oven to 400°. On a lightly floured surface, roll dough to a 1/8-in.-thick circle; transfer to a 9-in. pie plate. Trim crust to 1/2 in. beyond rim of plate; flute edge. In a large bowl, beat egg. Beat in sugar, flour and vanilla until well blended. Gently stir in rhubarb and strawberries. Pour into crust. For topping, in a small bowl, combine flour, brown sugar and oats; cut in butter until crumbly. Sprinkle over fruit. , Bake 10 minutes. Reduce heat to 350°; bake until crust is golden brown and filling is bubbly, about 35 minutes longer. Cool on a wire rack.