Unfortunately you would not be able to simply switch out the 2D CAD engine. In order for VeSys to work with AutoCAD it uses the Autodesk libraries (ObjectARX) this would not be compatible with any other CAD system without a change to VeSys Classic.
Given the fact that VeSys 2.0 is now available I would not anticipate that Mentor Graphics would want to make changes to VeSys Classic to run on another CAD platform.
I had a feeling that it was going to be the arx's that were going to stop me in my tracks.... Thanks Joel