Add egg yolks and the vanilla. Beat on medium-high speed until combined. Scrape down the sides and up the bottom of the bowl with a rubber spatula as needed. Beat in the vinegar and the food coloring-- until you reach your desired color. With the mixer on low speed, add the dry ingredients in three additions alternating with the buttermilk, beginning and ending with the dry ingredients, and mixing each addition just until incorporated. Do not overmix. Fold whipped egg whites into cupcake batter with a rubber spatula or wooden spoon. The batter will be silky and slightly thick. (If there are still pieces of butter, this is ok.)