In a large bowl, combine the oats, whole wheat flour, all-purpose flour, brown sugar, baking soda, baking powder, and salt. In a separate bowl, whisk together the beer, buttermilk, melted butter, and vanilla. Make a well in the dry ingredients and pour the wet ingredients into the well. With your fingers, stir the batter from the middle of the bowl to the outside, until it's well mixed. It will look more like cake batter than bread dough.
Brush a 9 x 5 x 2 1/2-inch loaf pan with melted butter. Pour the batter into the pan and sprinkle the top with oats. Put the bread in the oven, immediately turn the temperature down to 400 degrees, and bake for 45 minutes, until a toothpick comes out clean. Turn the bread out onto a baking rack and allow to cool completely. Slice and serve with salted butter.