Thanks Nano, yes I have things on separate layers. I think it's a problem with certain objects. Because exporting it to 3DS converts it to a mesh, things like cylinders cause a massive polygon blowout ie. lots of faces trying to approximate a cylinder. You haven't really got any idea of the polygon count until you try exporting.
The units are fine if you bring everything through from AutoCAD, but if you import extra models directly into Vue you have to be careful.
Last time I drew up some 3D boxes in AutoCAD that were the correct size of a car, bought them into Vue, & then scaled the cars in Vue until they fitted into the boxes.