Use Track Motion. That control is in the header area (extreme left) of video tracks. The icon looks like two folders overlaping.

Use the handles to resize videos to where you want them in the overall video space. While holding down your mouse you can drag into position.

I would take your basic idea one step or two further. Make three areas, lets call them windows or place holders. The larger would fill the top left, (main video) two smaller windows could be alongside to the right, below you could have text messages appear fading in and out at key moments or instead have the main window grow and shrink with the smaller windows taking over for a short time, then fading back. All fairly simple to do with Vegas.