Crystal Bresley sends us a tip about the command needed to change the shape of a viewport.
"Have you ever created a regular viewport, only to realize later that you wish it had been created as polygonal so you could manipulate its vertex? Using the Vpclip command allows you to quickly change your viewport to polygonal while saving your viewport settings at the same time. Simply select the viewport you wish to edit, and then retrace."
Notes from Cadalyst Tip Patrol: There are many times when a rectangular viewport will suit your needs. Sometimes, however, you need an irregularly shaped — or even round — viewport. When you use Vpclip, you are given two options: select a polygon that is already present to be the new shape of your viewport, or create a polygon to be the new shape of your viewport. Either will serve your purpose just fine.