The present application is directed to cap assemblies and particularly, cap assembly for closing an opening in a vessel, the cap assembly including a stopper including an polymer body adapted to fit an opening of a vessel, the stopper also including a tubular portion which defines an internal passageway extending through the polymer body; and a rigid cap attached to and integral with the stopper, wherein the cap is adapted to engage the vessel and provide a sealing force between the stopper and the vessel.