Combine the flour, salt and butter in a large bowl. With a bench scraper or pastry cutter, cut the butter into the flour until it forms pea-size chunks. Slowly add the ice water, folding it into the flour until it comes together in a dough; be mindful not to overwork the dough. If the dough is too dry, add another tablespoon of water at a time. Turn the dough out onto a lightly floured work surface. Divide in half and form into disks. Wrap the two disks in plastic wrap and refrigerate for at least 30 minutes to let the dough rest.
Preheat the oven to 425 degrees F.
Roll out one of the disks into a 12-inch round on a lightly floured work surface. Roll it up onto the rolling pin and carefully transfer to a 9-inch pie plate. Gently press the bottom crust into the plate with your fingers. Leave the edges untrimmed.
To the bottom crust, add the rhubarb, strawberries, sugar, cinnamon, flour, salt, lemon juice and vanilla. Dot with pats of the butter.
On the same lightly floured work surface, roll the second disk of dough into a 12-inch round. Lay the top crust over the pie and crimp the edges.
Brush the top crust lightly with milk and sprinkle with granulated sugar. Cut vent holes in the center. Bake for 20 minutes; lower the temperature to 375 degrees F and bake for another 45 to 60 minutes, or until the filling is bubbling.
Cool completely before serving to allow time for the filling to set up.