I am allergic to both dairy products, and yet many common recipes include butter as an oil, or as a creaming agent in dishes such as mashed potato.  Depending on your allergies you may be able to use an oil based substitute, but being allergic to soy means that many dairy alternatives are also not suitable.  If you are in a similar predicament I suggest you try using coconut oil.

Nature's Way Efagold Coconut OilI have tried several different products and have had the best cooking success with the Nature’s Way EfaGold coconut oil shown in the photograph above.  This is a food quality coconut oil, looking like solid white shortening or lard at room temperature.  It does not need to be stored refrigerated, and has a good shelf life.

In small quantities it will act as an excellent butter substitute even when used on boiled vegetables, or sweetcorn.  However, be sure to practice to see how much you like, as at a certain point the coconut taste of the oil passes from unnoticeable to overwhelming.  It should also be noted that coconut oil can have a natural laxative effect if taken in larger quantities.