Preheat oven to 350°F.
Pull all wet ingredients (eggs, buttermilk & butter) out 30 minutes - 1 hour prior to baking to allow ingredients to come to room temperature.
Spray a 9"x13" pan with regular cooking spray or baker's spray. If you have parchment paper on hand, you can line the bottom with that and then spray it with cooking spray. In a small bowl, mix buttermilk, vinegar, and red food coloring together. Set aside.
Place butter, vegetable oil, and sugar in a mixer and beat on medium-high speed for 2 minutes until light and fluffy. Scrape the bowl.
Add in eggs & vanilla. Beat for 1 minute until fully incorporate and homogenous. Scrape the bowl.
In a medium bowl, sift flour, cocoa powder, salt & baking soda together. Give the flour mixture a whisk until all is incorporated.
On low speed, add the dry flour ingredients to the cake mixture in three parts and buttermilk in two parts, starting with the flour and alternating with the buttermilk. ie: Flour mixture, buttermilk, flour mixture, buttermilk, flour mixture.
Mix until fully combined and no streaks remain. Using a rubber spatula, scrape the bowl to ensure all flour and buttermilk have fully combined.
Pour and spread the red velvet cake batter into your greased 9"x13" pan. Bake for 25-30 minutes. Your cake is done when a toothpick is inserted and comes out with a few moist crumbs.
Cool in the pan for at least 1 hour prior to frosting.