Whisk together the cake flour, cocoa powder and salt.
In a separate bowl, beat the sugar and oil with an electric mixer. Add the vanilla, food colouring and eggs one at a time, beating well with each addition. Add the flour mixture in 2 additions, alternating with the buttermilk.
Combine the baking soda and vinegar in a small bowl, then add to the batter and mix well.
Pour into 2 greased and floured 9-inch round cake pans and 1 square 9-inch cake pan. Bake at 350F for 30-35 minutes, or until a skewer inserted into the centers comes out clean. Cool completely.