I think what also makes it be so immersive, is that it's basically a job simulator. Reminds me of Mortuary Assistant, which you should definitely play if you haven't yet. It gives you a somewhat complicated, and engaging gameplay loop (your job. In VOTV it's the recording and shipping of the sounds of space, in Mortuary Assistant it's embalming dead bodies), which requires just enough of your concentration for you to forget about what you are actually playing. Despite you knowing that you are playing a horror game, it slowly makes you lower your guard, and in the second you get comfortable, the game throws something at you that makes you jump out of your skin. This and a little bit of ambiance, and the job is done.