I just read your last post on here, I think we were talking about 2 different things. The anti siphon on the trap arm is the one I'm talking about. The ones that go on the tailpiece I think provide a good answer to floor drains that have a dry trap problem.
They do not show pictures of their products, but state that the "NovaVent" comes in sizes from 3" to 24" which would not be appropriate for a lavatory drain.
I'm no plumber but I'd go with the previous guys solution. It appears to have worked and it's in the vanity so no one sees it.