It is a tough decision. The vapor barrier might be a really good idea...or it might not be needed at all... .....and how the heck do you tell without putting up a wall and leaving it in on one side and not the other? If you do need the vapor barrier, you've got to protect it from punctures, which adds great expense to the project(don't think you ever planned on wooden floors?) Are your existing coops dirt or wooden floored? That might tell you what the amount of moisture that might rise from the soil?