Prick pastry shell with fork.Bake @ 450 degrees for 5 minutes. Cool.Combine rhubarb with sugar, flour and salt.Let stand 15 minutes.Beat eggs slightly.Add vanilla.Stir egg mixture into rhubarb.Turn into pastry shell.For Topping;stir together flour and sugar.Cut in butter until it resembles coarse crumbs.Sprinkle over pie.Bake in 425 degree oven for 15 minutes, then reduce heat to 350 degrees.Bake for 30 minutes more.Cool completely before serving. Store pie in refrigerator.