Combine 1/3 of the cake mix with 1 egg and margarine in bowl, mixing well. Press into greased 9x13 inch pan. Bake at 350 for 15 minutes. Combine remaining cake mix, 3 eggs, brown sugar, corn syrup, and vanilla in a bowl. Beat for 2 minutes with electric mixer on medium speed. Pour over crust. Sprinkle with pecans. Bake for 30 to 35 minutes or until light brown. Cool and cut into bars. Enjoy!