Which gas tank?

I am not home near my pc to look at those photos. There is no canister on the car. So plugging the little vent tube on the tank is not good???
He had this tank from a 69 dart on here with no vent at all. I am so confused. So plugging it off could cause problems?
I have no vent now at all. The gas cap is original. I dont know if the cap is vented or not. Would this be a really bad thing then to plug it? If so. I guess I can rig something up.