Cream the margarine, shortening, and vanilla together. Slowly, add in powdered sugar and beat slowly with mixer.
Splash in milk, milk alternative, or water to reach desired consistency.
Notes
This came from a cake decorating class and has never failed me.
Substitutions
Butter and Margarine: Butter is a dairy product made from cow’s milk. Margarine typically contains milk or soy, but there are milk-free and soy-free versions available.
Corn Substitutions: Corn is a common ingredient in products. Starch, modified food starch, dextrin and maltodextrin can be from corn. Consult with your physician to find out which corn derivatives you need to avoid. Many corn-free options are available in the US. Find out more about corn substitutions.
Milk and Soy Substitutions: Alternative dairy-free milk beverages and products will work in most recipes. Find out more about milk substitutions and soy substitutions.