The demon Vassago appears in the grimoires Book of Incantations, Book of Treasure Spirits, The Book of the Office of Spirits, The Book of Oberon, and The Lesser Key of Solomon
From The Lesser Key of Solomon, Crowley/Mathers edition:
Vassago
The Third Spirit is a Mighty Prince, being of the same nature as Agares. He is called Vassago. This Spirit is of a Good Nature, and his office is to declare things Past and to Come, and to discover all things Hid or Lost. And he governeth 26 Legions of Spirits, and this is his Seal.
This demon has the same nature as the demon Agares