Basically, Vital Souls can affect and rewrite the Grimorio, which is one with the whole world, however, this is far from being any plothax shit. The whole thing is about being capable to rewrite the world by materializing what is the book into reality. You are not literally changing the narrative of Castlevania, you are simply changing the universe with Subject Reality
You are changing the narrative. The world of guys is literally a book that has the whole story in words. And using the powers of Chaos, the beings literally change the narrative. Making the heroes who originally won a fight, now lose as the narrative was changed.
And I think you got it wrong. The grimoire and the "real" world are different things. Changing what is written in the grimoire does not change the real world at all, but the world of the grimoire. As I said, the grimoire is not a book that if you write something in it or change something, it changes the events of the real world. You change the grimoire world itself.
The Grimoire and what is written on it would be the very reality of the beings that are part of the grimoire.
Imagine there is a crystal ball, and this ball is one with the universe in the sense that everything reflected in it is what represents the world
Consequently, a wizard is able to project any illusion into such a ball thus can turn such an illusion into reality
You would definitely not call this as plothax
But what if instead of a crystal ball it was a book? Now it would be plothax?
1 - You are misunderstood. The grimoire is not a book that is one with the real world, and if you change it, it changes reality. The grimoire world and the real world are different. The grimoire would be something like the very structure of the reality of those beings. It's like our Universe/Timeline is literally a book with its narrative written in words (Obviously, since it's a book)
2 - To be plot hax, metafictional elements are necessary, and a crystal ball, being a universe or something, does not have these elements. In the case of Grimorio, it is literally a book with a written narrative. During the various crts about the castlevania plot hax it was discussed whether it was just reality manipulation or plot hax, and it was plot hax because of the metafictional elements. So if in your example, the crystal ball was literally the fabric of reality and it was actually a book with the story written in words, and changing the words changed the story itself, then it would probably be plot hax.
Anyway, if you disagree, make a crt. Arguing here will lead nowhere, and it's not even the right place for that kind of discussion.