Novels and non-5E products are officially considered non-canon for the development of 5E D&D products, though the latter may draw inspiration from older game products and novels.
https://www.enworld.org/threads/wotc-no ... on.681553/
https://comicbook.com/gaming/news/dunge ... me-novels/
That pretty much settles the question as to what caused all the changes to known characters in the various Domains of Dread in VRGtR and CoS. Nothing. They were intended to serve as a full reboot of Ravenloft, and to stand apart from material associated with previous editions of the game.
So no need to beat yourself up about how to mesh the changes in VRGtR into the pre-5E timelines... you now have the official blessings of WotC to do your own thing!

That said, I do like that they clarified what they consider canon or non-canon when developing 5E products, regardless of campaign setting.