"Realms of Power: Magic", but it's not generally a problem unless you've got a lot of magi extracting lots of Vis frequently and I find the aura degradation tedious to calculate so I tend to just ignore it.
Also, Sahir have plenty of other ways to obtain Vis such as visiting Mazaran or travelling to the Magic Realm (Intellego and Warping) See "A Passage to Strange Lands"