Hello friend. Vassal is actually not misspelled, and is a different word from vessel. In this context, the vassal of horror would be an emissary or owner of lands of horror.
I agree that "vassal of horror" is an unusual turn of phrase but the amount of arguing about this post online just about that phrase alone is staggering. It almost makes wonder if the original note was written by a AI-bot since it's the exact kind of unique phrase that -sounds good- but hasn't come up organically yet in any other published work.
But that's the opposite of what AI tends to do. It tries to predict words that fit a pattern. It wouldn't come up with a new turn of phrase that sounds good because it isn't creative. It has no idea about what "sounds good." It would use a phrase it's seen many times before, because that's what it trained on. Often that ends up being incorrect, but it's written like a normal human would write because it's literally copying what they wrote. It isn't creating new stuff.
I have no idea why people are being so aggressive to "correct" you. You're almost certainly right. Horror is not the liege of this bug. It is a thing which contains horror. It's a vessel of horror, not a vassal of horror.