VP doesn't have garbage mattes, but you can suggest the feature. https://www.nch.com.au/suggestions/index.html?software=VideoPad
You might get creative. For example, take these two images... 
The background is 16:9. The girl's green screen isn't that wide. If chromakeyed (green screen effect) the BG will fill only the green area.
To remove the side-bar black 'garbage' areas and cause the background to fully appear, study these effects. Click the image to enlarge it.
Two chromakeys, one for the green, the other for black. The girl shot has an outside mask applied to the green screen area.
