Pulling air out of the vivarium is a great idea. Just keep in mind that you will have to do more to provide humdity, as the fan will draw humid air out of the vivarium. Humid air tends to drop, so place the fan as high as possible.
On the other hand, blowing air INTO the vivarium is not a good idea. Drafts can cause several problems. I don't have any fans near my terrarium, but I do make sure the room that my veiled chameleon is in is well ventilated.