Do not preheat oven. Grease and flour a 10 inch tube pan.
In a large bowl, cream together the margarine, shortening and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Beat in the flour alternately with the milk. Pour batter into prepared pan.
Place cake in cold oven, then set the temperature to 300 degrees F (150 degrees C). Bake for 90 minutes, or until a toothpick inserted into the center of the cake comes out clean. Let cool in pan for 10 minutes, then turn out onto a wire rack and cool completely.