Hi,
If you use LabVIEW's built-in source code control, then you can either put all the VIs on a network drive that both the outside and inside developers can access or else go by what you yourself suggested i.e.
copy the entire SCC setup from the outside developers to your in house system periodically and clean it all up manually.
Otherwise you may want to consider third pary Source Code Control systems. LabVIEW can interface with several SCC systems. You can visit the following link for more information on how you can use Perforce with LabVIEW.
http://zone.ni.com/devzone/conceptd.nsf/webmain/69EA27204D24F78586256E2400773ABE
Regards,
Ankita A.