Preheat oven to 400 degrees. Grease an 8-inch square baking pan. In a large bowl, stir together cornmeal, flour, sugar, baking powder, salt, and baking soda. In a medium bowl, whisk together melted margarine, soymilk, white vinegar, and eggs. Add milk mixture to flour mixture and stir just until combined (do not overmix). Transfer batter to pan and smooth top.

Bake until a toothpick inserted in center comes out clean, 20 to 25 minutes. Let cool in pan on a wire rack, 15 minutes, before serving.