In the bowl of a stand mixer, combine butter and sugar and mix on medium speed until very light and fluffy, about 5 minutes. Add the eggs, one at a time, beating well after each addition. Add food coloring, cocoa powder and vanilla together; add to the butter/sugar mixture and mix well. Add the salt into the buttermilk and add to the batter in three parts, alternating with the cake flour, starting and ending with flour. In a small bowl, stir together the vinegar and baking soda; add to the batter and mix well.