Prep oven and pan. Preheat oven to 350°F (175°C). Grease a 9x5 inch loaf pan thoroughly.
Make the swirl. In a small bowl, whisk together the 0.33 cup sugar and 1 tbsp cinnamon. Set aside.
Mix the batter. In a large bowl, combine the flour, 1 cup sugar, baking soda, and salt. In a separate bowl, whisk the buttermilk, oil, egg, and vanilla. Pour wet ingredients into dry and stir gently just until combined.
Layer the bread. Pour half the batter into the greased pan. Sprinkle half the cinnamon sugar evenly over the batter. Carefully pour the remaining batter over the sugar layer, then top with the rest of the cinnamon sugar.
Bake. Bake for 45 to 50 minutes. Check with a toothpick; it should come out clean. Tent with foil if the sugar top browns too quickly.
Cool. Let the bread cool in the pan for 20 minutes before carefully turning it out onto a wire rack.