There are nearly a hundred videos on You Tube showing how to do a split screen effect, but this is the only video I've seen that I think shows a better alternative. And uses the tools Vegas has for this kind of thing.
While I applaud the youtuber's creativity and understanding of compositing and masking, the only tools required for creating split screens are Pan/Crop and/or Track Motion. Of all the screen segmentation efforts I've ever done, the subjects of the two to n clips have rarely been in the right place on the first try. In spite of any filters applied, it was necessary to move or resize each of the videos.