In a blender or food processor, finely grind oats. Transfer to a large bowl. Stir in flour, brown sugar, 1/2 cup pecans, the baking powder, baking soda and salt. Make a well in the center of the dry ingredients. In a medium bowl, combine buttermilk, eggs, 1/4 cup melted butter and the vanilla. Add the milk mixture all at once to the flour mixture. Stir just until moistened (batter should be lumpy).