I must have been over excited after discovering the using the trim command on a roof object because I just trimmed a viewport and thought the result was cool enough to post. I turned the viewport frame red to show it up better. Notice the RHS (Right side image) is missing. But it still works just fine.
Also you can match properties to get an adjacent viewport to follow scale and layer settings (inside the viewport) as well as the expected behaviour. So for e.g. if you already have a viewport set up and decide you wanted to use a special shape instead of the std rectangle, you can create your viewport shape, zoom in to match closely the view scale you want and then (moving to paperspace) matchproperties to get the new viewport set exactly as the old including layers. Once created, you have the freedom to move the corners around to reshape it. Want to add more points? Just select the viewport, right click and select the Viewport Clip option - Polygonal and redraw a new viewport boundary with extra vertice if you require.
(BTW the viewport I trimmed was created using the above PLine (object) option in case you just tried to trim a regular viewport :-)