In a medium saucepan, combine the raisins, water, sugar, cinnamon, and nutmeg. Bring the mixture to a simmer over medium heat and cook for about 5-7 minutes until the raisins begin to soften.
In a small bowl, mix the cornstarch with a couple of tablespoons of cold water to create a slurry. Stir this into the raisin mixture and cook for another 2-3 minutes until the filling thickens. Remove from heat and stir in the butter and vanilla extract.
Preheat your oven to 375°F (190°C). Roll out the pie crust and fit it into a 9-inch pie pan. Pour the raisin filling into the crust, spreading it out evenly. If you’re using a top crust, roll out the second piece of dough and place it over the filling. Crimp the edges of the crust to seal it. If you like, brush the top crust with a beaten egg for a golden finish.
Place the pie in the preheated oven and bake for 40-45 minutes, or until the crust is golden brown and the filling is bubbling. Let the pie cool for at least 1 hour before slicing.