Combine rhubarb and strawberries in a big bowl. Combine sugar, flour and scraped vanilla bean seeds and mix in with fruit mixture. Let sit while rolling pie dough.
Preheat oven to 425 degrees F. Butter a 9-inch glass pie plate. Roll 1/2 pie dough to approximately 1/8-inch thick and about 1 inch larger than the pie plate. Fit in into the pie pan leaving the overlap intact. Fill with fruit filling and dot top with 2 tablespoons of unsalted butter. Brush beaten egg with pastry brush around the rim of the pie. Roll out the remaining dough and make a lattice top. Crimp the edges. Brush lattice with beaten egg. Do not brush the edge of the pie--this will cause the dough to become too dark while baking. Generously sprinkle with crystal sugar. Bake for 10 minutes. Lower oven temperature to 350 degrees F and bake for an additional hour. Pie is done when golden brown and juices bubbling from pie appear thickened.