Preheat the oven to 350 degrees F. In a large bowl, mix the flour, baking soda, sugar, and salt; set aside. In a saucepan, bring the oil, margarine, peanut butter, and 1 cup of water almost to a boil (do not boil). Pour over the dry ingredients and mix well. Add the eggs, vanilla, and buttermilk. Mix well. Pour the batter into a greased and floured 15x11x1-inch pan. Bake for 15 to 18 minutes, or until a toothpick inserted into the center comes out clean.