VAC comes from CSGO.
My friend got one of his accounts stolen, so I added his second account on my steam guard so that it won't get stolen too. This dumbass got lazy of sitting in gold 4 so he decided to start hacking. He disabled steam guard on his account so that I wouldnt get VAC because of him. He got banned and so did I for some reason. I guess it's just faulty VAC/Steam guard system over at Valve...