All you need for some condensation is a moisture source and the side of the metal away from the moisture to be cooler than the other. The metal cools off, moisture condenses out of the warmer air. That is simple physics. How moist the air is and the temperature difference make a a big difference.
In a cold climate with a poorly insulated coop where it is a little warmer inside and moisture is available from their breathing and poop, you can get condensation underneath, for example. Good ventilation will usually take care of this. Usually.
Do you need a vapor barrier? Depends on how you build it and your conditions. Where I am, if I keep good ventilation and don't put anything under the metal roof, it is not a problem. I've lived a lot of different places but I'll admit I do not have a lot of experience with this in all those places.