Julian D, The easiest way to convert an autocad model for 3ds is to make sure that all of your parts are on seperate layers, as long as no single layer exceeds that limit then you should be just fine.  I've been exporting models up to 30 mb with no problems by splitting the objects up into layers.  The real units thing is annoying but as long as you're always working in the same units (I typically use the architectural units as those seem to be the closest to "real world" as vue gets, then the model should always come in to relative scale to the other objects (provided you have "resize and center objects" unchecked.  Also moving the model to 0,0,0 is needed as well when importing into Vue (as surveyman pointed out)