"According to some (respectable) people, VAC isn't even loaded on MP due to issues with TAC (TreyArch Anti Cheat)."
On google "tac anti-cheat"
Totally agree with them... half of players are cheaters!
Even, it doesn't make any sens when vac banned on record are playing on secured matchmaking servers.
Even with 2 ip forwarding on my router to my pc, the game is unplayable... Aimboters everywhere.
In 600 hours of match, 3 where without cheaters, always against them, loosing at 70 per cent even always first of my team with a general k/d of 1.