Preheat the oven to 325 degrees F (165 degrees C). Lightly grease a 9x13-inch baking pan.

In a large bowl, cream together the sugar and margarine until light and fluffy. Stir in the eggs one at a time, beating well with each addition, then stir in the vanilla.

In a large bowl, sift together flour, baking soda and salt. Blend this mixture into the egg mixture, alternately with the buttermilk; stir just to combine. Fold in the mashed bananas; mixing just enough to evenly combine. Pour batter into prepared pan.

Bake in preheated oven until a toothpick inserted into center of the loaf comes our clean, about 60 minutes.