Can you have a default vas.conf file put in place when you join?
Yes you can accomplish this with a group policy. You add the vas.conf settings to a group policy so they can be pushed out to a machine. A policy must be linked to the OU or container that the machine is in. So you can edit one that is already linked or add a new one to the OU that the computer is in.
Open Microsoft Group policy editor, Expand Computer configuration, Unix Settings, Quest Authentication Services, Client Configuration, the double click on QAS Configuration and pick your vas.conf settings.
On the client machine to apply a group policy right away run the vgptool apply command. To view what group policy are applied to this machine run the vgptool rsop command.
There is more information about Managing Unix Hosts With Group Policy in the AuthenticationServices_4.0_AdminGuide.pdf which should be in the docs folder of the download or available on the Support Portal website.