Beat 1 1/4 cups sugar, sour cream, eggs and 2 tablespoons flour in a large mixing bowl until smooth. Stir in vanilla and salt. Fold in rhubarb. Pour rhubarb mixture into pie shell.
Bake for 30 minutes in the preheated oven. Meanwhile, combine 1/3 cup sugar, 1/3 cup flour, and cinnamon in a small bowl. Cut in butter with fork or pastry blender until mixture resembles coarse crumbs. Set aside.
Remove pie from oven. Reduce oven temperature to 350 degrees F (175 degrees C). Sprinkle topping mixture evenly over pie. Return pie to oven and bake until filling is set and crust and topping are golden brown, about 30 minutes.