Substituting Soy milk +vegan butter for whole milk +margerine for whipped fluffy icing
Yes. Soy milk (or any other non-dairy milk such as almond milk) can be used as a direct substitute for dairy milk. (Your mileage may vary if you make homemade soy milk, which may have a more "beany" flavor.) In addition, vegan margarine such as Earth Balance can be used as a direct substitute for butter. In fact, these are two of the easiest vegan substitutions.