Preheat the oven to 350 degrees F (175 degrees C). Grease a 9 inch square baking dish.
Place the rhubarb and strawberries into a bowl. Stir in 2/3 cup of sugar until fruit is coated. Pour into the prepared pan and spread out to cover the bottom. In a medium bowl, beat the remaining sugar with the butter until light and fluffy, about 3 minutes. Mix in the egg, orange zest and vanilla. Combine the flour, baking powder, baking soda and salt; stir into the batter, alternating with the buttermilk. Pour the batter over the fruit in the dish.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 55 minutes.