Vectorworks has some really cool 3D tools, and it's tempting to always look at these…
Vectorworks Tip #264 – Site Modeling – Site Modifiers – 3D Polygon
Intermediate Tip – 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.