That vent on mine in the basement is hooked to nothing, I see no purpose for it since the walls do not go all the way to the top of the basement anyway there is lots air going in and out of the basement to the underbelly so what's the point of the Vent
if you mean the cold air coming from under the stairs I just put filters behind the stair vents and that helps keep the cold air out of the rig without impeeding air flow much