Combine first 6 ingredients with a fork until crumbly. Press into a greased 13x9-in. baking dish. Combine filling ingredients; mix well. Pour over crust. Bake at 350° for 50-60 minutes. In a bowl, beat egg whites until stiff. Gradually add sugar and vanilla, beating well. Spread or pipe over hot filling. Return to the oven until lightly browned, 10-15 minutes longer.
If using frozen rhubarb, measure rhubarb while still frozen, then thaw completely. Drain in a colander, but do not press liquid out.