Videomaker – Learn video production and editing, camera reviews › Forums › Technique › Editing › Multiple Videos on One Screen › Track Motion and keyframes
Track Motion and keyframes are the tools you want to use for this.
Remember that the top track has priority so place your central video on the top track.
There's a short video (4 min.) on the basics of Track Motion on the Sony training page at http://www.sonycreativesoftware.com/training/vegaspro