If it's a USB version you're dealing with, then it won't be accessible through a COM port layer.
I just looked here:
http://www.cs.unc.edu/Research/vrpn/
and it says VRPN doesn't yet support the Spaceball 5000.
Tell you what, we'll try writing a native Vizard plug-in to support the Spaceball line. Someone here can start working on it Monday and we'll let you know by Wednesday if it's done.