Prepare macaroni according to package directions, In small mixing bowl,or saucepan, depending on whether you are going to microwave or not, combine soymilk, vegan cheese slices, and soy margarine. Microwave or melt cheese/soymilk mixture for about 2 minutes to melt the cheese and margarine. If using stove method, stir or whisk constantly until melted. Bring just to a boil. Remove from heat and either using a hand blender or other hand mixer, on low speed, mix till creamy. Add desired seasonings. This sauce is really creamy, and will set quickly. If it sets before you use it, simply reheat for about 1 minute. Pour over macaroni and stir. Serve. (Very rich)
This sauce can probably be used as a base for any cheese based sauce, like alfredo... just add appropriate seasonings.
Some tips: If you want a nacho style, use taco style seasoning and add some hot sauce and some chilis. Serve with Tortilla chips.