Preheat the oven to 325 degrees F. Spray the a bundt cake well with nonstick spray. Set aside.
In a stand mixer fitted with the paddle attachment, combine the butter, granulated sugar, brown sugar, and red velvet flavor. Beat the ingredients on medium-high speed for about 5 minutes, until the mixture is light in color and slightly increased in volume.
Lower the speed and add the eggs, one at a time, mixing until each egg is incorporated before adding the next.
In a separate bowl, sift the flour and cocoa powder. Sprinkle the salt on top of the dry ingredients and whisk until combined.
With the stand mixer on low speed, alternately add the dry ingredients and the buttermilk into the butter mixture, beginning and ending with dry ingredients. Mix until just incorporated. Fold in the sour cream.
In a small bowl, combine the vinegar and baking soda until it fizzes and then add to the batter. Stir until blended.
Bake for 55 to 65 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs on it.
Let the cake cool completely in the bundt pan before turning it over to remove the cake.