Is Saint William of Vercelli a patron saint?

The date is unknown. It is quite possible that William was never canonized as the official canonization process was not in place at the time of his death. He could have been elevated to sainthood by popular acclamation and approval of a local bishop.

Pope Alexander III - William is not included in the list of saints canonized by Pope Alexander III nor does the Patron Saint Index list a canonization date for him.