Preheat the waffle iron according to manufacturer's instructions and preheat the oven to 200 degrees F.
Combine the flour, pecans, baking powder and salt in a medium mixing bowl and whisk to combine. In another bowl, combine the egg yolks and sugar and whisk until pale yellow. Add the vanilla extract, 1/2 cup melted butter, and milk, and whisk to blend. Combine the egg and milk mixture with the flour mixture and whisk until just combined. Do not overmix.
In the bowl of an electric mixer, beat the egg whites until soft peaks form, about 1 minute. Using a rubber spatula, gently fold the egg whites into the waffle batter. Do not overmix.
Using a pastry brush, lightly coat the waffle iron with some of the vegetable oil. Pour enough batter into the iron to just barely cover the waffle grid. Close the waffle iron and cook according to manufacturer's instructions, or until golden brown, 4 to 6 minutes. Transfer to a baking sheet and keep warm in the oven while you prepare the remaining waffles.
In a saute pan, add the remaining 1/2 cup of butter and the pecan pieces. Cook, stirring occasionally, until pecan pieces are light golden and fragrant, 2 to 3 minutes. Add the banana slices and cook until light golden and soft, 2 to 3 minutes. Add the syrup and bring to a simmer. Serve with the warm waffles.