a half cup of medium-firm or silken tofu whipped smooth would would nicely as long as you upped the baking powder by a bit (up to 2 tsp?).
you could also substitute 2 tbsp soy milk whipped with one tbsp of starch (corn, tapioca, potato, arrowroot) for each egg, upping the baking powder by the same amount you would if you used tofu.
baking powder is baking soda mixed with some starch, as well as an acidic powder (often cream of tartar).
replace the butter with earth balance or another vegan margarine.
it should work out fine.