Using softened butter in place of margarine shouldn't be a problem, only that it won't be vegan anymore. Even ghee (clarified butter) in semi-solid state shouldn't be a problem, if you like the flavor. You won't have to make any changes to the recipe. But if you want to use oil, the recipe has to be tweaked a little bit. I'm estimating that 1/3rd cup of oil should be enough instead of 1/2 cup of butter/margarine.