I think if they had meant to base this off of the VOCALOID song series, they would have used the characters in place of their own people. For example, they could have used Meiko for gluttony, since that is her song in the Deadly Sins series. I understand your viewpoint, especially since I am a VOCALOID fan myself, but the Deadly Sins are a common theme outside of the VOCALOID fandom. (sorry if I sounded harsh, that was not my intention. o-o)