In a bowl, mix flour and baking powder.
In a 1-quart glass measure in a microwave oven at full power (100%), or in a 1- to 1 1/2-quart pan over medium heat, warm soy milk with 1/4 cup margarine until margarine is melted, stirring occasionally.
In another bowl, with a mixer, beat eggs, sugar, and vanilla until foamy. Add flour and milk mixtures. Stir to mix, then beat until well blended.
Lightly coat 2 cake pans (each 9 in. wide) with margarine. Pour batter equally into pans.
Bake in a 350° oven until top of cake is browned and springs back when lightly pressed in the center, about 25 minutes (about 20 minutes in a convection oven). Invert cake from pans onto racks. Serve warm or cool.