I was in a public match playing with a friend as 3 of his clan mates. I was spectating one of the members on my team before I was kicked and got the message "VAC banned from secure server". I thought it was a mistake so I joined a new server and got the same message.
I have no hacks, didn't mess with any of the .dll files or anything like that. Turns out one of My friends clan mates had the exact same thing happen to him in the same game. I wouldn't hack in a game I put money into in risk of being banned for no reason. I do believe VAC made a mistake.