This item is used to cover removed (unscrewed / filed) vents covers.
A fake vent cover can be reused, however it has a durability limit. Every time it is removed, 20% durability is lost. You can recover it by pressing the right mouse button.
If the vent in your cell isn't covered by either a real or fake vent cover, you'll be sent to solitary confinement, removing most progress towards escaping which you have made so far.