Oh, it's more of a technical problem then:
So you need to get close to the vent and interact with it with the "E" key, a dialog will appear and soon after the first Living Milk Pudding will appear, after dealing with the Living Milk Pudding you should get close to the vent again and interact once more with the "E" key, another dialog will appear and the once closed door will now be unlocked!
If this does not work, I will need more details on your problem (screenshots, videos, anything useful).