Vitkir & Eternal Life?

You may be right. Perdo Corpus aging is too easy to be balanced unless it can be undone, though PeCo magic is over once the spell ends, so the aging is almost identical to 'natural' aging. Probably InCo magic can discern between natural aging and magically inflicted aging even if InVi magic couldn't pick up the residue of magic from the PeCo spell that did it.

Sounds eminently reasonable. Of course, I imagine that if winter has passed in between the aging and the fixing, and aging points have accrued, that probably can't be undone.

grin Maybe it fails completely, because it works completely, including against itself...

I like interpreting that effect as an "Invisible to Magic" spell. That is, the vitkir can't be targeted by spells and supernatural effects, but can still be affected by magical substances. That nicely dodges the Limit of Magic Resistance and still feels useful and fun.