Does your agreement/contract with the vendor include you providing original CAD files?  If not, don't send them.  Any time you transmit original CAD files to someone else you run the risk they can change something you are responsible for without your knowledge.  Unless you feel okay with this you will always be better off sending drawings in PDF or DWF format. 

If Autodesk really wanted to support a CAD editing lock feature they would do so.  But they haven't, and that's enough for me to protect original work unless it's being shared with parties I trust.  My advice:  Don't rely on 3rd party vendors to protect your critical drawings.

Dave.