Did Vidic hit Cahil's static hand or did Cahil move his hand up and Vidic hit it?
If it's the former then it wasn't a penalty, if it was the latter, then I'm afraid it's a penalty. Cahil might not have seen it, and he didn't, but by moving his hand like that in the penalty box, knowing that there is a guy behind him who can hit the ball and score or assist, he wasn't careful, it shows intention, and penalty should have been given.
I don't really remember what happened.