When it comes with relations between Visual Novel companies and Manga publishers, Kadokawa Shoten and sister company ASCII Media Works have the closest relations with them. Don't know why others like Kodansha, Shueisha or Shogakukan don't publish manga based off Visual Novels. It's probably because the majority of the Visual Novel companies are Hentai companies, so I guess that those latter three do not want to associate with companies that engage in the adult entertainment industry.