Is this for a Gmod server?
I really didn't think Gmod needed VAC.
Sure, might help keep out cheaters that would use aimbot apps and modified Steam files for CSS and other games, but Ive not heard of any applications like that written for Gmod.
Lua aimbots maybe, but not actual compiled apps and modified files.
Then again, I'm not in the loop like I used to be. Loopy perhaps, but not in the loop.
Also, I think VAC has to have a special port forwarded for Steam/hlds to call home to mom for security.
If Jay's fix doesn't work, let us know or look at the steam support site 'what ports need to be forwarded?'