Vectorworks Tip #264 - Site Modeling - Site Modifiers - 3D Ploygon http://learn.archoncad.com/wp-content/uploads/2015/05/image112-300x144.png

A 3D Polygon can be a site modifier if you assign it to the correct class. The important thing is the class, if you do not assign it to the class, it will not work as a Site Modifier.