Any oil or fat can be used to saute the vegetables for soup. Butter, meat or poultry dripping, olive oil or vegetable oil, will all give their individual flavors to the soup. Margarine does not provide such a good flavor and low-fat spreads are not suitable for frying because many of them are almost half water.
After sauteeing the vegetables, a little flour can be stirred in to soak up any excess fat and form a roux, and the resulting soup should not be greasy. If you do end up with a greasy soup, pour or spoon off any oil that comes to the surface, or if you have time, chill the soup so that the fat solidifies on the surface and can be easily lifted off. For a fat-free version, the oil or fat can be eliminated and the vegetables just simmered in the stock.
** Asian Recipes