As of the last few days, Valve initiated a huge VAC ban wave. Especially against a cheat provider known as "Ragehack" that was asumed to provide un-detactable cheats to hundreds of players. Their cheat was considered undetactable for up to 20 months. Although for how far back VAC actually started detecting it is confidential by Valve and one can only speculate.
VAC have how ever always had a huge delay in their bans to make it harder for cheat providers to experiment what cheats are detectable and which ones are not. Fact is, they truly caught many cheaters by surprise. Many of these hackers had 1000+ hours of gameplay, one can only speculate how many of these hours they have been playing legit. For example this guy:
This was posted on the cheat providers' forums recently:
It certainly feels good to be legit ^^