Mix flour, sugar, baking soda, baking powder and salt.
Cut in butter.
Add eggs, buttermilk and vanilla; stir well.
Put half of batter into a greased 9x13 inch pan. Spread rhubarb-strawberry filling on top. Spoon remaining batter on top.
For topping, combine sugar, flour and butter. Sprinkle on top of cake.
Bake for 40-50 minutes.