When you're dealing with Vectorworks there really is very little difference between DXF and DWG,…
Vectorworks Tip 412 – DXF/DWG Units
If you get the units wrong when you’re importing a DXF/DWG file, you will end up with information that is the incorrect size. This can be fixed, but it is better to get the units correct. One of the techniques I use is to check the scale settings to see if the scale looks about right for the units chosen.