Vegetable shortening is a product made by injecting hydrogen into vegetable oil to make it solid at room temperature and to give it culinary qualities that are similar to solid animal fats. This produces a partially-hydrogenated oil, which is laden with trans-fats – a very unhealthy kind of fat. Like butter, shortening adds air as well as fat so that substituting liquid fats will change the texture of the dish. That said, you can find substitutes for the very unhealthy fats by combining different fats – liquid and solid.
You can substitute EVOO or expeller pressed seed oils for vegetable shortening in any recipe that calls for melted shortening or that uses shortening to grease pans, at a 1:1 ratio. In a recipe that calls for shortening in its solid form, you should replace it with butter, coconut oil, or a mixture of EVOO/butter/coconut oil.