Vanilla is a timid person. Despite starting off as a pure witch, due to having awakened only shortly before the battle with Graycastle, she does not feel any hostility to other witches and joins the Witch Union after the Church's defeat.
At some point, Vanilla would have been adopted by the church in their efforts to gather witches. She awakened shortly before the battle with Graycastle and was kept by Zero in her strategy to absorb Roland.
Vanilla is one of the few witches Zero did not sacrifice to make more God's Punishment Warriors as she was needed to locate Roland's position during the battle. After the battle was finished, she was discovered quivering alongside a passed out Isabella and Margie.
During Isabella's interrogation, Roland reveals to her that he allowed Margie and Vanilla to join the Witch's Union since they had never directly caused anyone harm.
Vanilla joins Mystery Moon in her second detective case although Lily believes that she was just too nervous to say no.
Scent marking & tracking.
Vanilla was a pure witch for a very short time before the Church's downfall, and it is unclear how attached she was to it at the time.
Vanilla joins the Witch Union during Roland's two month coma. She is not often seen interacting with the other witches, but she does seem to be getting along with them as evidenced by Mystery Moon going to get her for the second detective case.