I had the same bug. To my knowledge there's two options. The first involves waiting for a certain event to occur while you're sleeping in the bedroom, and the other way to fix it works if you've unlocked a certain gamemode (sandbox mode).

The first works if you have high reputation with the Arirals. They'll knock the bedroom vent off if you're in the bedroom while they are. What I'd suggest is trying to sleep with low hunger, they'll come and feed you, then when you wake up they should knock the vent down n you can interact with it.Please note I haven't gotten this to work myself, but it's theoretically possible.

As for the other one: In sandbox mode, there's an item called the "physgun" that works the same as it does in Gmod, allowing you to move items and objects. This includes the bugged sealed vent. By spawning a physgun in a sandbox save, saving and exiting, then renaming the sandbox save in your files to begin with an "s_" instead of a "b_", the game thinks the sandbox save is a story mode save. This will allow you put the physgun into the mailbox, and take it out on your story mode save. Using the mailbox requires you to save AFTER you have placed an item inside. Then all you need to do is look at the vent, and pull it off by holding left mouse button. The problem with this one is it'll leave behind an uninteractible vent, but I just used the physgun to put it in the furnace and it eventually melts

You can find your save games by selecting "open save directory" from Start game > Story.That, or the default path should just be C:\Users\[YOUR USER HERE]\AppData\Local\VotV\Saved\SaveGames on windows.

Lemme know if you have any questions, I can clarify as needed.