As you know, AutoCAD file formats keep changing from year to year. Vectorworks file import/export follows these changes but it takes them some months each time. So if you are using an earlier version of Vectorworks you clearly can't expect it to open .dwg file formats that were not then in existence.
Practically this is hardly ever an issue, because you can always ask to have the AutoCAD file saved in an earlier .dwg format which you can open with your Vectorworks. This compares most favourably with the cost of paying the AutoCAD bill every year.
