Preheat oven to 400 degrees F (200 degrees C). Grease a pie pan.
Mix 3 cups flour, olive oil, ice water, and salt in a bowl until a dough forms. Place half of dough in the prepared pie pan, pressing down to form the bottom crust. Roll out the remaining dough in a circle wide enough to cover the pie pan.
Melt margarine in a small saucepan over medium heat. Stir in 3/4 cup flour to form a thick paste. Season with garlic powder, cayenne pepper, salt, and black pepper. Add soy milk and stir until combined with the paste. Bring to a boil; remove from heat and stir in mixed vegetables.
Pour vegetable mixture into the pie pan. Top with reserved pie crust.
Bake in the preheated oven until pie crust is golden brown, 30 to 60 minutes.