After getting rid of Krampus the vent is visible.
If you haven't yet picked up all the references to the movie Die Hard I suggest you go see the movie now and then come back.
This is the low-spoiler guide for "Vent" puzzle in The Darkside Detective. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.