Using a stand mixer or hand beaters, whip the margarine until fluffy.
Slowly add in the icing sugar until it's combined well.
Add in your vanilla.
At this point, check your consistency. Margarine isn't as dense as butter, so my amount of rice milk is very small. Is your icing nice and whippy at this point? Try maybe a slight teaspoon of rice milk. I chose rice milk because it's virtually tasteless, allowing the vanilla to flavor the icing instead. I would not use soy because of the after taste you can get with it.
Once you are happy with the consistency, place in the refrigerator until you are going to decorate.
Notes
There are TONS of vegan icing sugars, try Red Path, Trader Joe's, do your research and find one! I use Red Path.