VAC and Punkbuster are architecturally very different and use different theories of how to combat cheats. VAC basically takes your mod client DLL source(after you hand them over to Valve) and runs them through a 'scrambling' algorithm during compile that makes them very difficult to insert hooks into. Combined with STEAM (which is supposed to push core client files on demand to prevent you replacing them with hacked ones), and you have a pretty tough system to cheat on, theoretically. This is nothing like PB at all. Valve didn't steal anything, they just had a different direction.
This comment was edited on Dec 2, 10:45.