User blog comment:B214/Light Novel Volume 16 spoilers/@comment-162.202.198.72-20131025222020/@comment-9339850-20131025222433

Mythology in DxD is usually original to the series, so whatever you are referencing might not be true in the series. So as for Vali being Odin's adopted son, it looks to be only to get rid of his status as a terrorists, so he can be more useful without causing consequences for those that try to affiliate with him; so its just a political move, Because Vali isnt evil, per se, he just wants strong opponents to fight against.