X windows will not start on a vas-enabled system when it is not connected to the network, so the console login screen does not appear, making it impossible to log into the server.
This is a problem at the O/S level that can have many causes. Anything that prevents the display manager from starting can cause this, including errors in pam files, errors in inittab, wrong display settings or missing files.
The solution in the case seen so far was to re-image the server and re-install QAS. If that is not possible, then it may help to boot the machine into runlevel 2 or 3, and try and start X from the command line with debugging enabled.