Preheat oven to 180C. Butter and flour two 23cm round cake pans, or three 20cm round cake pans.
Sift together the flour, baking powder and salt into a medium bowl. Set aside.
In a small bowl, mix food colouring and cocoa powder to form a thin, smooth paste. Set aside.
In a large bowl, using a hand mixer or stand mixer, beat butter and sugar together until light and fluffy, about 3 minutes.
Beat in eggs, one at a time, then beat in vanilla and the red cocoa paste, scraping down the bowl frequently with a spatula. Add one third of the flour mixture to the butter mixture, beat well, then beat in half of the buttermilk.
Beat in another third of flour mixture, then second half of buttermilk. End with the last third of the flour mixture, beat until well combined, making sure to scrape down the bowl with a spatula.
In a small bowl, mix vinegar and baking soda. Add it to the cake batter and stir well to combine.
Working quickly, divide batter evenly between the cake pans and bake at 180C for 25-30 minutes, or until a skewer inserted into the centre comes out clean.
Cool the cakes in their pans on a wire rack for 10 minutes. To remove the cakes from the pan, place a wire rack on top of the cake pan and invert, then gently lift the pan. Allow cakes to cool completely before icing.