Notes:

Like the Redditor at the link, I too have come up with the theory that Vaggie is secretly an angel (the heavenly demon-smiting kind, not the gay spider kind).

The fact that she has a big "X" over one eye, just like the angels in the intro, is a tantalizing clue. And if she was an angel it would answer a lot of other questions about her:

1. The wing-like shape of her hair (supposedly she's a 'moth-demon' but she has no other moth-like attributes)
2. She's only been in Hell since 2014, yet (a) she's risen quickly up through the social ranks to become the concubine of the Infernal Crown Princess (a meteoric rise comparable to Alastor's own), and (b) she has an encyclopedic knowledge of Hell's history
3. She owns an angel's spear (not remarkable in and of itself, but placed in context it becomes more important)
4. For a denizen of Hell she is remarkably free of sin: she is a teetotaler (!!!) for one, and although she is quick to anger, she only ever does so out of a desire to protect Charlie, so it does not count as sinful Wrath.
5. It would also explain why she got in Alastor's face, even knowing how dangerous he is – perhaps she knew she could hold her own if they threw down.

My theory is that she is an angel who got sick of the Cleanses and wants them to stop. She defected to Hell in secret and made common cause with Charlie (Charlie probably doesn't know Vaggie's an angel). It would also fill in one small plot hole:

6. If what they need is a successful test case, then why doesn't Vaggie volunteer for the procedure? She wants to help Charlie, and this would be the most common-sense way to do it. But it wouldn't work if she had no sin because she was an angel…