In Vegas POST, is there a way to create a mask with reference to what is on the preview display?

E.g., I have a primary video track that I am placing a secondary video on top of with an element that has been shot on a green screen and has been keyed out. The primary track has a couch and a coffee table where I want to place the secondary element on top of and make it look like they are sitting on the couch with their legs behind the coffee table itself.

How I am doing it now is putting a Picture-in-Picture effect on the secondary video element, moving and sizing them where I want in the scene of the primary video element, then creating a mask to make it look like their legs are behind the table. This would be easier if I were able to mask with reference to the primary scene beyond creating the mask and looking off to the side where the preview is and seeing if it lines up.

Is there a better way to do this where I can mask directly in the preview window? Or maybe a way to have the primary video track show up as the background as I create the mask in the pan and crop window?