Can anybody explain why a magi can only produce Vim Vis in extraction ? (page 94 5th ed)
Obviously if you could extract any of the other forms it would alter the whole 'covenant secures vis sources' parts of the saga. May impact on those adventures to those areas by eliminating the necessity to hunt out & secure vis sources.
But mechanically i can't find any reason why you couldn't.
Well, there are ways to extract other types of Vis from auras, but they require special virtues. In any case, I believe the logic is that, since Supernatural auras normally fall under the auspices of Vim, any vis distilled from said auras would also be aligned with that Art.
I figured that it's Vim because you're extracting the vis directly from a "generic" magic aura. Thus, you are pretty much plugging into the raw power of the magic realm (but nothing that's "colored" by a form). There is the "Extractor of (Form) Vis" virtue in the RoP: Magic book which allows you to extract vis of a particular Form from an aura where that form is exemplified.
Now, there are other ways to extract form vis from an Aura, one is a Mystery Virtue (TMRE), and one is a Hermetic Virtue from RoP:Magic. Both of these are extremely limited in the amount that they can harvest in other forms. Extractor of Form can only distill his specific Form vis if he is in an aura that exemplifies that Form, he can produce 1/10 of his CrVi lab total in Form vis. The Hermetic Alchemy Mystery Virtue distills, 1/20th, rounded up, of his CrVi(Requisite Form) lab total, although this lab total can be modified by Shape and Material bonuses, that is negligible when dividing by twenty, but it might make the difference of getting an extra pawn of vis..
Its the most closely aligned to the power of raw magic. Vim is the form you use for creating or destroying auras. (Not that Hermetic Magic can do that yet.) That said, a breakthrough could allow different types of vis to be extracted, or for that matter eliminate the distinctions between sorts of vis completely.
I think that it behooves the game to have vis as a whole in short supply. That way one can structure stories around getting it. Also if vis is rare it will play a bigger part in stories and be more meaningful. It is more fun to have your characters quest after the mythic rheingeld then to go to their closet and remove a rook of terram. (I'm, in fact, in favor of making vis transfer moderately more difficult so as to keep vis in its original form more often and therefore better tie it to its origin instead of having it quickly moved into a ring or some such divorcing it from its story).
But trading vis can lead to nice stories too. Having to always run a vis finding adventure whenever the characters need more vis can be a real drag. So let them extract vim vis from their aura and only vim vis.
Vim is the most broadly useful type of vis and it is the most tradeable (is fungible the word I'm looking for?).
Vim vis is the only type of vis that can be used to open items for enchantment
Vim vis is one of the two types of vis that can be used in the Aegis of the Hearth Ritual
Vim vis is always usable in a longevity ritual.
A character can "make" vim vis in the lab and trade it to get what he wants. If you can only choose one art (which I think is a good choice from the point of making good stories) then vim is the art to go with.
Naturally this is an explanation created in hindsight to justify things. There's no way to know if the original authors put the same thought into it.