Running cable through vapor barrier
Daniel Griscom said (in a comment): 


  I doubt the barrier is perfect enough so that you'd notice the effect
  of a couple of small holes in it. If so, then a good quality duct tape
  might do the job (depending on the vapor barrier material).


I agree with Daniel, but I would use RTV (room temperature Vulcanization) to seal the hole as tape usually fails with wires in my experience.