Do not preheat the oven. Grease and flour a 10-inch tube pan.
Cream together sugar, margarine, and shortening in a large bowl with an electric mixer until light and fluffy. Beat in eggs one at a time, mixing well after each addition. Stir in vanilla. Add flour alternately with milk, beating briefly after each addition. Pour batter into the prepared pan.
Place in the cold oven, then set the temperature to 300 degrees F (150 degrees C). Bake until a toothpick inserted into the center of the cake comes out clean, about 1 1/2 hours. Let cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely.
