Remind me to try this one day. With the right setup that could make you pretty much invincible. 
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No drawback... Except that there's a minimum possible Might cost for Ritual effects (unlike Greater, Lesser, and Personal effects, whose costs can be reduced to 0), meaning that they cost permanent Might, because Ritual. Of course, if you don't have much to worry about in regards to vis, you could keep forcefeeding them to get their Might back up.
Make your familiar immune to might stripping (that's real?) and you will have plenty of antagonists who would be more than willing to kill it and render it for Vis. Rendering for Vis, the ultimate might stripping.
Through the Aegis, page 156, box, Agnes, Guardian Spirit.
Greater Immunity (Perdo Vim).
Neatly solves both questions of possibility and value of virtue, neh?
Confidence Points work, as does vis if you have a high confidence score and Vis Mastery.
Well, I was curious as to it stipulating might stripping specifically. If Perdo Vim is indeed an immunity, Perdo Vim magics would still counter effects that she generates, because Perdo Vim acts on the effect, and not the caster. So, for all intents and purposes, it is Greater Immunity (Might Stripping).
But the value of said virtue is still questionable, at best, when destructive Hermetic Magics can still easily penetrate and cause damage.
I believe so, yes.
Why would that be? Ablating, for example, isn't PeVi. I suspect the Greater Immunity is supposed to be that (against Might Stripping), but that's not what was written. Also, regardless of "Perdo Vim" or "Might Stripping," Acclimation is neither of those.
Greater Immunity Hermetic Might Stripping then...?
It might fall under the "if it's not Hermetic, shoehorn it into the appropriate TeFo for purposes of figuring out what it is" rule for determining magic resistance. As such, "Perdo Vim" could reasonably be shorthand for "any magical effect that would be categorized as a PeVi effect".
I do agree that it's not clear, though.
We could ask the author what was intended?
The intention was to have a spirit immune to simple might stripper spells. However 'might stripping' is not an official term in ArM5 rules and mechanics, it is a word used more casually and by the fan base at large here on the forum.
Hence the term Perdo Vim is used for the immunity. This may even be a bit broader immunity, however I fail to find any examples of another Perdo Vim effect used on the spirit itself.
Should another creature have a power to 'suck might' or otherwise destroy it, I would see this as comparable to the Perdo Vim the spirit is immune to. Other non-Hermetic powers like Ablating also fit the bill IMHO.
The Verditius inner Mystery Bind Magical Creatures and the Mystery Virtues Spell Binding & Hermetic Empowerment use ReVi, ReVi(Fo) or Re(Mu)Vi(Fo) to drain Might. Granted, they're not "simple", but they'd work despite the PeVi immunity, wouldn't they?
How are requisites factored in when dealing with a TeFo Immunity? In this case is there a difference between how the Immunity interacts with say PeVi, Pe(Re)Vi, Re(Pe)Vi or PeVi(Fo) spells?
A Vim based Demon with Greater Immunity (Rego Vim) could be a nightmare for the Order, though.
I don't think there is a general rule, since I believe Agnes is the first such publication.
There might be others, but I couldn't point one out to you.
If I recall correctly (that's a very, very big if) there are some folks in tC&tC who have a Greater Immunity to "mind control", including all of ReMe, skill use such as Charm, and possibly PeMe effects that e.g. calm emotions.