This is like the opposite of the Add Surface and it also makes it easy…
Vectorworks Tip #191 – Basic Tools – Clip Surface
Like the Add Surface command, you can access this from the Modify menu or from the contextual menu. As with the Add Surface command, I prefer to access this from the contextual menu (right-click). Use this command to subtract a shape from the base shape. With this command, the stacking order is important. If you get the stacking order wrong you will be clipping the wrong object.