Now since Hermetic Magi don't have Might, you cannot directly extract Vis from them.
However, the fan-fic The Mirarion set in I assume Ars campaign with some major House rules shows a possibility.
I am not certain how the Breakthrough might be accomplished, but lets assume somebody finds a way to indirectly get Vis from a magus.
Say during the normal practice of extracting Vis from an Aura, if you filter it through the corpse of a Hermetic Magus, a number of pawns of the extracted Vis are Corpus rather than Vim. Perhaps 1 pawn per 5 levels of Corpus score (round up) of the deceased Hermetic Magus.
I would say yes, but not through regular hermetic means - it's a pretty gnarly thing. Look up the Ablating rules from ROP:Infernal page 115+, whereby you can extract vis from "spirits". You could consider that to be souls too (I am). I would treat any activity attempting this as that, even if the character is not aware it's infernal - they might just have to discover it when too late. Tempting hunger for power and all that...
I read and liked the story, too. I think that nobody would harvest the vis from a sodalis, except in the most dire circumstances. If you could harvest vis from hedge wizards who are not part of the order, the question of how much vis a human life is worth would come up, and the answer might well vary for each individual magus.
I'd say, read up on diablerie in Vampire the Masquerade and it should give a fairly good idea.
Turning to how it might be possible, I am more inclined to allow harvesting vis from a spirit or soul than from the corpse. Hermetic powers being linked to the spirit or soul rather than they body, could explain why no vis is left in the corpse by default. Like @OriginalMadman I think Infernal powers are better suited than Hermetic ones. I am not at all convinced that the Gift matters to the endeavour. I would think you would be harvesting the life force, rather than the Gift of magic.
IIRC, in the story the amount of vis you could harvest from a body depended on the Corpus score of the magus.
I wouldn't allow a hermetic power to interfere with the immortal soul. If you want to harvest it from the spirit, you would first have to make sure the magus leaves a spirit after dying, then you'd have to trap the spirit.
Given any reasonable Corpus<=>Vim exchange rate, they're much more valuable to you alive extracting Vis from the local aura in conditions of abject slavery. Renewable sources, people!
I am presuming that the Art of Corpus saturated the mage's flesh in such a way that it can be used to help extract a limited amount of Hermetically recognisable Corpus Vis.
Addenda - as part of the Season long Vis extraction from Aura.
I feel on a rules angle, this is firmly put in the SG discretion pile. Extracting vis from a Magi falls in the corpse defiling part. A games company just doesn't publish those kind of things.
It seems logical a Magi corpse can have vis extracted. If we can extract vis from intelligent monsters, then it should be possible to extract vis from a magi corpse with no breakthrough necessary. I appreciate Magi do not have a magic might, however, there is such a high level of magic on Magi, that it effects "normal" people around them. An experience Magi can do magical effects, greater than nearly any magical creature.
I think extracting vis from a Magi corpse is already possible, and may have been done in the Schism War. In peaceful times it is not done by any but the most debauched Magi, and is a high crime that would result in a wizard's march.
I'd imagine the vis would be in the heart or brain, so the process of extracting the heart or brain would be grisly, and wandering around with a shrunken brain or heart is pretty bad form. Saying all that, there are some magi who would do this.
Numerically, I'd think get the fallen Magi's highest technique form combo, divide by 10, that is the vis. Depending on how vis rich or poor the campaign is, one could divide by anywhere between 5 and 20, I'd think.
Going to the original question. Filtering via a corpse. That's even more grisly. How long is the magi keeping this body? That's next level horrible.
When another Magi sees this, he could kill the corpse filtering magi, no war declared, and get complete absolution from any tribunal, and probably get a vis award from a grateful tribunal for ending the actions of a psychopath who could bring harm to the order.
That is true, and for very important reasons. If it is made explicitly possible, it encourages players to take up a practice so disconcerting that it threatens to derail any saga who is not explicitly set up with rails in this direction. It should be reserved for only a few sagas to explore it.
This is why original research is as loosely defined as it is. On the rare occasion that a player (or NPC) decides to go this way, it is possible, but the difficulty can be adjusted to have it happen at the right time of the narrative.
Maybe, if we assume that one or both sides used infernal powers. Hermetic Original Research takes too long, so it would only be developed for the next war and not the current one.
On the topic of filtering, this seems awfully close to Hermetic Alchemy, which allows Shape and Material bonuses to affect vis extraction (and imbues the resulting vis into an item).
It seems plausible to me that an unsavory practitioner might realize that the corpses of magi provide excellent Shape and Material bonuses for this purpose. Alternatively, an infernally Tainted version of Extractor of Corpus Vis that has this requirement might be the culprit.
Finally, a magum with Chthonic Magic could probably defile a corpse in this manner for a lab total bonus.
Another possibility is that it was already known at the start of the war how to extract vis from the corpses of magi - some esoteric knowledge hidden away in a book of which only a few copies existed, that was so grizzly that nobody expected it to be of actual use until the war. After the war, there was likely a mutual understanding to never talk about that sort of thing ever again. Veterans don't usually like to talk about the atrocities that were committed during a war. So this little piece of information would have been stricken from the historical records.
My opinion is that you can't actually get Vis from the corpse of most mages, neat though the story implications are: they use magic, but don't accumulate it. I have no solid reasons for this, it's just my feel for how the world works: no Vis without Might.
I say "most": I think it's plausible that you could get Vis from someone with a personal Vis source or with Imbued With the Spirit of (Form), to take a few examples. (The latter explicitly says it someone with that probably consumed a creature with Magic Might, after all.)
And if you want more body horror, with the right power you could probably get Vis from a Leper Mage as long as you didn't kill them.
"It is possible to extract vis from the bodies of the Gifted. It was a technique known to the druids, which is why the Cult of Mercury used all its influence to push Rome to mercilessly annihilate them. It was for fear of the technique that the Founder Diedne was so insistent that every druid then living join the Order or die. The true reason for the Schism War was House Tremere learning about its existence as a Mystery of House Diedne, and bringing the proofs of its continued use to the Quaesitores.
"All evidence that the technique ever existed was suppressed after the war, even to the point of the use of Mentem magic to destroy the memories of its existence in all surviving magi -- except for records magically sealed to succeeding Primi of Guernicus and Tremere. Those exceptions, which contained not enough to use the technique, only identify the evidence of its use, were only allowed to ensure the technique could be swiftly re-suppressed if it were revived. The suppressors went to such lengths to hide that such a technique was even possible in the hope that it would save the Order from anyone rediscovering such an incentive for preying on ones sodales.
"That I am telling you this, of course, means that their efforts failed."
-- Prima Poena to the Exarchs of House Tremere, on the Ides of April in the 454th year of the Order of Hermes, and of our Lord the 1221st.
(Incidentally, that speech works even if in your saga House Diedne was completely innocent, wrongfully persecuted by House Tremere with the cynical acquiescence of the Quaesitores. After all, how would anyone prove that Poena is lying about the Schism War? The speech itself explains the absence of any evidence that House Diedne ever used the technique. A little slander of the long-dead, and the reputations of House Tremere and the Quaesitores get a little extra shine while rallying the Order to deal with a new crisis.)
So out of curiosity: people who think you can get Vis from a dead mage, where are the lines on this? Presumably not from ungifted Redcaps? What about a gifted Learned Magician? Or an ungifted one? Someone with the Gift but no magical abilities? An apprentice who's just had their Arts opened? A Hermetic mage with no warping yet?
I'm not trying to catch anyone out here, I'm interested in how people think this works.
"I've developed a new technique to convert warping into vis, though there is one little drawback."
"That's great, we must get this out to everyone immediately! It will revolutionize the Order! Oh what's the drawback?"
"The magus with the warping dies."
"Does it have to be a magus?"