I'm surprised there's nothing about this, because it seems pretty important to know: What happens after?
As suggested in RoP: Magic, this not only varies on the type of creature, but on whether or not they have 0 or non-zero Might.
Can Might be recovered after a reduction spell is leveled?
Can a being rendered mundane by PeFo become magical once more? Or are their essential traits enough of a basis to restore them?
What are the mechanics for recovering lost Might, if any? Is it the same as for using a Ritual power?
No one else has replied so there's probally not a rule to quote, but I'd have them recover might as you would from wounds (assuming they have the correct aura and anything else needed). If it's a ghost that gets laid to rest then I'd call it dead, but if it just gets oblivioned then it'll probally reform after some time. Strip the might from a nasty creature and it'll be just a bit less nasty but now it's really mad.
I don't think you can make it mundane without a lab effect such as the verditius and vis harvesting from creatures.
As I noted in a post on a similar topic, in my view, a creature which is PeVi out of existence is "dead" in the sense that it no longer interacts with the mundane/Hermetic world. Whether it is "destroyed" in a universal sense is more of a cosmological question, and I think the rules have been vaguely inconsistent on the question.
As for wounded... the impression I get based on some of the rules in RoP:Magic, is that Might taken via PeVi isn't like a wound... it's like maiming. Realm Creatures could probably regain lost power by consuming vis, but that's pretty much the same as how they grow mightier in general, so it's not quite like "healing".