I had students do a level-test and producing DWG & VIZ Render files. I can open them directly in VIZ Render and all is fine. Didn't had any problem with it. These files were copied on a server, so they weren't located in the original place.
I must say I've had the interface lockup once... We upgraded ADT2004 to SP1 so that might be able to solve some issues.
You can assign a different DWG from within the File Link Manager (but it apparantly doesn't work with non-2004 DWG files, probably to prevent using VIZ Render without AutoCAD ADT 2004).
We also upgraded to ADT2004 + VIZ Render to replace ADT3.3 (well, we only used the AutoCAD 2D+3D stuff) and VIZ r3. I'm not finished with our tutorials, but it might be interesting to exchange some ideas.
--- stefkeB ---