This term is rarely used to mean magical. In fact, as much as it means a “barer of burders” and it is a title given to Aaron the helper of Moses in the Qoran. In modern day it is essentially used to mean Royal Court’s secretary for the last 2000 years, sometimes with some devolved political power from royalty. Often in Pantomine and the odd novel, the Vizier is a deceptive person manipulating the power of the throne, often behind some bumbling, incompetent Saltan. They sometimes are depicted as sorcerers, but it is not a part of the definition. A perfect version of this is Jafar from Aladdin and from a Thief of Baghdad.
I have never heard someone introduce themselves as a Vizir in a magical conference. The fact it appears in Oberon Zell Ravenheart’s Grimoire for the Apprentice Wizard makes me worried he doesn’t know what he is talking about. The word that most Arabs would use to mean Sorcerer would be Sahir or, occasionally, Hukm which means sage, but is used in a modern context to mean a judge. An example of this would be Gayat al-Hakim (plural of hukm) which means the Stone of the wise (i.e the philosopher’s stone).
See also other types of Magical Practitioner.