You can get around the 4-track limit in VMS by compositing, but it takes a little extra work. In my case, I need more than 4 video tracks maybe only 10% of the time, so I'm willing to get around the limit when I need to, and save spending the extra $ on Vegas pro.
If you want an further explanation on what I mean, let me know. If however, upgrading to Vegas Pro is still what you want to do - enjoy!