I would make the creature sick after vis extraction. If you drain more than half of his might, it might die. At least IMS and in case we allowed it.
The drain would be permanent: you are draining the magic-ness of the creature, turniong it more mundane. Some creatures might give you part of their vis willingly, but that would be along the lines of an ordeal, like a magical badger (castor? the animal that makes dams in rivers) giving you his testicles for fertility and creo vis, or a magical crow with Visions, removing one of his own eyes. Those do not regrow, and the creature becomes less powerful because of his ordeal. It is a permanent scarring.
Xavi