I think Sheryl is a clone, which really works with her playing up her "all natural" appeal. Irony FTW.
I'm still sticking with the Vajra being a human creation, perhaps an unintended byproduct of early experimentation with "Fold Quartz". In a way, they are humankind's own Protodeviln. As far as the Ranka/Aimo connection, well, I'm not entirely sure. I could guess that her mother had something to do with the creation of the Vajra, and perhaps the Vajra have some genetic memory of an earlier, more peaceful time involving Ranka's mother and that song. But, I dunno.