Hidden Deep Playtest: How to Pass the Vent Part

Ascend a little above the vent. (You can ascend even more than on screenshot)

Start to swing. (I still don’t know, it also works with small swing too.)

Release. I recommend pressing C rather than space, i don’t know, but i think it will increase your chances to fail. Then, press W+D (don’t spam too much, otherwise you get stuck and die)
IN SHORT:
Release the rope by pressing C (by default)
Start pressing W+D (Don’t spam too much)

Here as you can see i stood up while in the vent, which led me to death.

…If you did everything right, then you are in vent! Congratulations!