Hi Vaquero,
I'm also a Phasespace and Vizard user. I'm happy to chat sometime if you would like. I haven't seen the issues you're seeing with scaling factors, but your'e right that Phasespace's system is in mm and Vizard's in meters.
I have a script that I'm willing to share that you may find helpful. It does not work through VRPN, but through Phasespace's own Python scripts.
http://cis.rit.edu/performlab/contact
- gD