Prepare the oven and pan. Position a rack in the center of the oven and heat it to 350°F. Grease and flour a 9-inch by 13-inch baking pan.
Prepare the cake batter. In a mixing bowl, using an electric mixer, combine the cake mix, baking soda, buttermilk, vegetable oil, eggs, 1 teaspoon of vanilla, vinegar, and food coloring. Beat on low speed until well blended. Increase the mixer speed to high and beat for an additional 2 minutes.
Bake the cake. Pour the batter into the prepared baking pan. Bake for 25 to 30 minutes, or until cake springs back when lightly touched with a finger. (For cupcakes, divide the batter into lined muffin tins and bake for 18 to 20 minutes, or until a toothpick comes out clean.)
Cool. Transfer the cake to a rack and let it cool completely.
Make the frosting. In a mixing bowl, using an electric mixer, beat the softened cream cheese with the butter until the mixture is smooth and well-blended. Sift the confectioners’ sugar into the bowl and beat on low speed until blended. Increase the heat to medium and beat until the mixture is smooth and creamy. Beat in 1 teaspoon of vanilla and add warm water as needed to achieve a soft, spreadable frosting.
Frost the cake. Spread the cream cheese frosting over the cake and sprinkle with chopped pecans or coconut, if desired.