Preheat oven to 350 degrees. Line the bottom of your cake pans and your stainless steel bowl with parchment.
Place butter, oil, sugar, and vanilla in bowl of an electric mixer and beat using your paddle attachment at medium speed until well-blended. About 5 minutes. Meanwhile sift and whisk flour, and salt into a bowl and set aside.
Beat in your 6 eggs two at a time.
Add flour mixture to your mixer in 4 parts, alternating with your buttermilk in 3 parts. Always start with dry ingredients first! Scrape down bowl and beat just long enough to combine.
Place baking soda in a small dish, stir in vinegar and add to batter with machine running. Beat for 10 seconds.
Place in oven and bake until a cake tester comes out clean. Let cool in pans completely before removing your cakes.