Preheat the oven to 425º F. Line a jelly-roll pan or baking sheet with foil to place the pie on before cooking; it will keep any filling from dripping into the oven and burning.
In a medium saucepan, combine the carrots, peas, potatoes and celery. Cover with water, bring to a boil and cook until the potatoes are tender, about 15 minutes. Remove from the heat, drain and set aside.
In a large skillet over medium heat, cook the onions in the butter substitute until they are soft and translucent, about 5 minutes. Stir in the flour, salt, pepper, celery seed and garlic powder. Cook for 2 minutes to get the flour taste out. Slowly stir in the broth and then add the milk. Reduce the heat to medium-low and simmer until thick, about 5 minutes. Remove from the heat and stir in the drained vegetables.
Roll out one of the unbaked crusts to a 13-inch diameter and place in a 9-inch deep-dish pie plate. Pour the mixture into the bottom crust. Roll out the second pie crust and place on top. Seal the edges and cut small slits in the top to allow steam to escape. Bake until the pastry is golden brown and the filling is bubbly, 30 to 35 minutes. If the top of the pie becomes too dark, loosely cover with foil and continue cooking. Cool for 10 minutes before serving.