Preheat oven to 350 degrees. Grease and flour two 9 inch cake pans.
Combine sifted flour and salt and set aside. Put the cocoa in a small glass bowl, and add the food coloring gradually, stirring until mixture is smooth. Set aside.
Cream together the shortening and sugar, beating for 4-5 minutes at medium speed in your electric mixer until fluffy. Add the eggs, one at a time, beating for at least 30 seconds after each addition. At low speed of your mixer, add the flour mixture to the sugar mixture alternating the buttermilk and vanilla, scraping down the sides of the bowl as necessary. Add cocoa/food coloring mixture, mixing until color of batter is uniform. Do not over-beat or it will result in a tough cake.
In a small bowl, mix the vinegar with the baking soda. It will foam up. Stir it briefly to mix, then add to the cake batter, folding it into incorporate will, but do not over beat.
Pour batter into the prepared cake pans, and bake in a 350 degree oven for 25-30 minutes or until cake tester comes out clean. Let layers cool on rack for 10 minutes before turning out. Let cake cool completely before frosting.