It's not just about using the same editing software.
There are many issues surrounding networked resourses, media management and such to be considered.

Also, not everyone working on a large project is editing.

CGI artists, animators, compositors are using a host of additional software. (After Effects, Combustion, Flint, Flame, Shake, C4D, 3dMax, Maya, XSI, and countless others...) Many of these applications are pre-configured (or have plugins available for..) to automatically support the high-end editing programs or the editors have clean, seamless support for the output from these additional programs.

Vegas just isn't there ..... at least not yet. (...and it may not strategically WANT to be either. Sony has XPRI Systems targeted at the higher end market. It's not a bad thing. There's pleanty of other market areas to shoot for with Vegas.) Again, I can't imagine that this has much bearing on the work of indie video professionals.