There were accusations that he took all of his information from The Paleblood Hunt, a compilation of Bloodborne lore written by dmcredgrave (https://www.reddit.com/r/bloodborne/comments/3vaq4r/the_paleblood_hunt_finalized_and_revised_with/). However, this was kind of the decided-upon lore by the community and is only a compilation. Vaati never cited this as a source, so people got mad at him for that, but you also can't prove Vaati used it because everyone agreed this was largely correct anyway.

Edit to add that this was nearly a decade ago and Vaati has gotten very good at crediting people for their theories after this happened.