Setting a Border for a Site Model

You can set a border for a site model easily in VectorWorks 12.5 and 2008.
Create a site model.

Trace over the site boundary, or some other shape that you want to use as a boundary.
Cut the shape that you want to use.
Select the Site model.

Go to the Menu Bar.
Choose Modify > Edit Group.
At this point you can see all the 3D loci used to create the site model.
Go to the Menu Bar.
Choose Edit > Paste in Place.
Click on the Orange button at the top right of the screen to exit.
You are back at the site model. The site model does not show your boundary yet.
Go to the Object Info Palette.
Click on the Update button to see the site model stopping at the boundary.