As bad as IWNET is, wasn't VAC supposed to be policing all the aimbotters and wallhackers?
VAC is good but because of its delayed banning system it really needs to be used in conjunction with dedicated servers and at the very least with admins, MW2 fails because it has none of those, it doesn't even have a proper check built in to make sure VAC is running. Valve themselves stopped people blocking VAC a bit but there's not much they can do because of its terrible implementation into the game.