In a large mixing bowl, combine the rhubarb, strawberries, and vanilla. Combine the 1-1/4 cups of sugar, cornstarch, cinnamon, and salt in a smaller bowl and stir.
Pour the dry mixture over the fruit, and toss gently with a rubber spatula. Brush the strawberry jam lightly over the flat part of the bottom crust. (This help keeps the bottom crust from getting soggy.)
Tip the fruit filling into the crust. Distribute the cubes of butter on top of the fruit. If desired, cut a vent in the center of the top crust with a donut-hole cutter, or simply make a few 1/2-inch slits in the top of the pie so the steam can escape. Position the top crust over the filling. Trim and crimp the edges. Brush the top crust lightly with milk and sprinkle with sugar.
Start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to 400 degrees F (or High) and preheat, lid closed, for 15 to 20 minutes.
Put the pie on a rimmed baking sheet. Bake for 50 to 60 minutes, or until the crust is golden and the filling is bubbling. (If the pie crust rim browns too quickly, arrange a small strip of aluminum foil around it.)
For the best presentation, let the pie cool on a wire rack for 2 hours before cutting and serving. (When the pie is warm, the juices will run.) But if you intend to serve it with vanilla ice cream, the pie should be slightly warm.