This edge is extremely costly and it is unclear exactly what
benefits you get from it. Is it both a PvE and PvP thing? Any
enlightenment on this would be appreciated!

Vivimancer's Rejection slightly increases the level of spells casted from devices against targets with no or abnormal metabolism (e.g., undead, elementals, etc.).

It also appears to boost the level of at least one offensive transmuter spell when casted against targets with wonky metabolism.

I wouldn't call this edge extremely costly. It's expensive, but right around where most of the class edges are.

nt