I am running Windows 7 64 bit Ultimate and using Cisco AnyConnect VPN 2.5.3041. As expected, Fast User Switching got disabled as soon as I installed the VPN software.
This FAQ from Cisco references how to enable Fast User Switching when their VPN product is installed:
A. Microsoft automatically disables Fast User Switching in Windows XP when a GINA.dll is specified in the registry. The Cisco VPN Client installs the CSgina.dll to implement the "Start Before Login" feature. If you need Fast User Switching, then disable the "Start Before Login" feature. Registered users can get more information in Cisco Bug ID CSCdu24073 (registered customers only) in Bug Toolkit.
My problem is that I have disabled this on the client, but fast user switching is still greyed out.
This article mentions a registry edit, however they key they mention, GinaDLL, does not exist at the WinLogon registry point.
This article from Cisco covering AnyConnect specifically gives a one liner:
AnyConnect is not compatible with fast user switching.
The only problem is I now I had found a workaround before the last reformat/reinstall, but I can't remember what exactly I did previously.