Preheat oven to 350°F. Grease a 9 x 5-inch loaf pan.
Whisk oil, sugar, egg, sour cream and vanilla in a large bowl. Whisk in buttermilk, then flour, baking powder, baking soda and salt. Pour half of the batter into prepared pan.
Combine brown sugar and cinnamon in a small bowl. Sprinkle ½ of the sugar mixture over batter; pour remaining batter evenly over top. Sprinkle remaining sugar mixture on top. Using a knife, swirl through batter.
Bake 50-60 minutes, or until a toothpick inserted in center comes out clean. Cool 10 minutes, remove from pan and cool completely. Wrap in plastic wrap overnight before serving.
