In a mixing bowl, combine flour and salt. Add cold butter and mix until the mixture resembles coarse crumbs.

Stir in ice water, a tablespoon at a time, until the dough comes together. Divide into two discs, wrap in plastic, and refrigerate for at least 1 hour.

In a large bowl, combine chopped rhubarb, sliced strawberries, sugar, cornstarch, lemon juice, and vanilla extract. Mix well and set aside to macerate for 15 minutes.

Preheat the oven to 425°F (220°C). Roll out one disc of dough on a floured surface to fit the pie dish. Place it in the dish and fill with the rhubarb-strawberry mixture. Dot with butter.

Roll out the second disc and cut into strips to create a lattice top. Weave the strips over the filling and seal the edges.

Place the pie on a baking sheet and bake for 45-50 minutes, or until the crust is golden brown and the filling is bubbling.

Let the pie cool for at least 2 hours before slicing to allow the filling to set.