Preheat oven to 400 degrees F. Line two baking sheets with parchment paper and set aside.

In a large bowl or bowl of a stand mixer, whisk together flours, oats, baking powder, sugar and salt. Cut in cold butter and knead with fingers or a pastry cutter until mixture resembles coarse crumbs.

In a separate large bowl, whisk together buttermilk, vanilla and eggs. Pour into flour mixture and stir until just combined. Add rhubarb and stir until just combined.

Dump dough on a well-floured surface. Flour a rolling pin and roll out dough into a 1-inch thick rectangle. Cut into quarters widthwise, then in half lengthwise. Cut each square in half on a diagonal to yield 16 triangles. Place triangles 1 inch apart on prepared baking sheets. Brush each with egg wash and bake until golden brown, about 20-25 minutes. Remove from oven and cool.

Meanwhile, make the vanilla glaze: Mix powdered sugar, milk and vanilla extract in a bowl until smooth. Add more milk or sugar as necessary to achieve a semi-thick consistency. Drizzle over cooled scones and serve.