Heat oven to 350 degrees.  Blend cake mix, pumpkin filling, mayo and eggs with electric mixer on medium speed until well blended.  Pour mixture into greased jelly roll pan.  Bake 18-20 mins. or until toothpick inserted in center comes out clean.  Cool on wire rack.  While cooling, prepare frosting, as follows:  Blend powdered sugar, softened margarine, milk and vanilla with electric mixer on low speed until moistened.  Beat on high speed until light and fluffy.  Spread over cooled cake.  Cut into bars.