Preheat oven to 350 degrees, and prepare a 9X11 or similar sized baking dish with butter or nonstick spray.
In a large mixing bowl, combine the flour, baking soda, baking powder, and salt; stir to combine and set aside.
In a medium mixing bowl, combine sugar and eggs, stir until smooth, then add in the vanilla and melted butter.
Gently fold the wet ingredients into the dry until combined and smooth. Pour into the prepared baking dish. Pour the strawberries and rhubarb into the center and gently spread into the batter. Bake in the oven for 40-43 minutes, or until the batter is golden brown and set.