Most icings have vanilla extract in them. I've never heard of anyone worrying about it. I've known alcoholics who have DRUNK extract when nothing else was available, but I've never heard anyone raise concerns about regular icing that is made with extract as a flavoring, spread all over a cake. It is such a tiny amount that you get in the icing, and the icing is spread all over the cake, that you aren't getting enough to affect you.