Heat oven to 350 degrees. Generously butter two 9-by-2-inch round cake pans. Sprinkle with flour, and tap out the excess; set aside. In a medium bowl, whisk cake flour, salt, and cocoa; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, combine the sugar and oil, and beat on medium speed until well combined. Add eggs, one at a time, beating well after each addition. Add food coloring and vanilla, and beat until well combined. Add flour mixture, alternating with buttermilk, scraping the sides of the bowl with a rubber spatula as needed.
In a small bowl, mix baking soda and vinegar until combined. Add to batter, and beat for 10 seconds. Evenly divide batter between the prepared pans.
Bake until a cake tester inserted in the center of each cake comes out clean, 30 to 35 minutes. Transfer to a wire rack to cool in the pans for 5 minutes. Remove from the pans, and return to the rack to cool completely.
If you want your cake to be even redder, increase the food coloring by one to three tablespoons.