I like using the Clip Cube; It is like a 3D viewport crop that is…
Vectorworks Tip 332 – Using a Clip Cube to Make a Section Viewport.
After you have set up your clip cube, you can use it to create a section viewport. Use the selection tool on the clip cube, and when a side of the clip cube highlights, right-click on it to bring up the Contextual Menu. At the bottom of the menu you will find Create Section Viewport…