My question might be silly but I could not found an answer by myself.
The main rulebook refers multiple times to magical creatures and faeries as potential vis sources. However I could not find the rules explaining how (and how many) to extract vis from a magical creature corpse.
Could you please let me know the page number if it is written somewhere?
There isn't a page number as such. However, if you look at the creature descriptions in the main rules, you will see that their statistics include a line that describes where the vis is. There is typically a pawn of vis for every five points (or fraction) of their Might. There are some special rules revolving around vis from creatures of different realms which enhance the descriptions in the main book; these can be found in the various Realms of Power books. However, these are just enhancements of the main rules and are not necessary.
If you would rather not carry around lots of animal bits, you can always transfer the vis into a more suitable vessel, as described on page 94 of ArM5.
As Mark pointed out, each magical creature has a specific amount of vis in its body, usually in some specific part, which must be extracted in a way specified in its description.
To make the whole extraction thing easier, you can use the ReVi 15 spell Gather the Essence of the Beast (ArM5, page 162).
Houses of Hermes: Mystery Cults also details some other procedures through which Verdiditus magi can extract Vis from magical creatures. IIRC (Noble's Parma!) it provides far more pawns of Vis than just killing a creature.
Not so much of an "extraction," as such. You bind the performance of the magical item to the creature's Might. As long as the creature is alive, your item works. Dead, the item loses it's magical power.
What is not clear in the HoH book, to me anyway, is (a) whether the animal can use any 0 might point cost powers, (b) whether a PeVi attack on the animal immediately kills the creature or dispells/disenchants the device, and (c) whether disenchanting the device causes the Might to flow back into the creature or whether it simply fades.
That all being said,using a ReVi spell to draw the vis from the creatures remains or move it into a more convienient vessel is the way to go for the rest of the Order.
IIRC (Serf's Parma): The Mage may extract a number of pawns equal to the Might of the Creature, providing his Rego Vim lab total is Double the Might of the creature.
The Mage may also Bind a magical power of a creature: This is similar to above, but only one power per creature type (Unicorn is the given example). There isn't any mention of "Death of the Creature", though one could certainly HR such a thing.
The drained creature may use any power that it has the Might for. In the case of a creature drained to zero might, it would only be able to use powers that cost nothing.
Perdo Vim would not kill the creature, unless it were a creature that did not have a body...IE A magical horse is still a horse...
No. Again there is no mention of this.
-No, it is pretty clear that the effect is "Permanent"