Beat butter and sugar till very fluffy and add one egg at a time. Add red food coloring and vanilla. In another bowl combine dry ingredients. In another bowl combine buttermilk and vinegar. Lightly blend flour mixture and buttermilk mixture into butter and sugar mixture alternating starting and ending with flour mixture. You can use 2 9" pans or 3 8" pans. Bake in 350 degree oven. To check if done, press your finger in center of cake. It should leave a slight dent. You will think it needs to go a little more but take it out. Keep in pan for 20 minutes. It keeps cooking after it is out of the oven.
For frosting heat milk and flour on the stove until you have a very thick paste. Cover with a piece of plastic wrap so that you don't get a skin on top. Cool to room temperature. Make sure it is cooled to room temperature before you go any further. Beat butter and margarine with sugar till well combined. Add vanilla and cooled flour and milk mixture and beat on high for a very long time till very fluffy. Frost cake and keep in refrigerator. Take out of refrigerator about 1/2 hour before you want to serve it.
I usually make the milk and flour mixture for the frosting in the morning because it has to be cool before you can make the frosting.