The demon Vual appears in the grimoires Book of Incantations, The Discoverie of Witchcraft, The Book of the Office of Spirits, Pseudomonarchia Daemonum, The Lesser Key of Solomon, and Dictionnaire Infernal
From The Lesser Key of Solomon, Crowley/Mathers edition:
The Forty-Seventh Spirit is Uvall, or Vual, or Voval. He is a Duke, Great, Mighty, and Strong; and appeareth in the form of a Mighty Dromedary at the first, but after a while at the Command of the Exorcist he putteth on Human Shape, and speaketh the Egyptian Tongue, but not perfectly. His Office is to Procure the Love of Women, and to tell Things Past, Present, and to Come. He also procureth Friendship between Friends and Foes. He was in the Order of Potestates or Powers. He governeth 37 Legions of Spirits, and his Seal is this, to be made and worn before thee, etc.
Timeline of related events
1536Publication of De praestigiis daemonum (lists this demon)
1563First known appearance of The Book of the Office of Spirits (lists this demon)
1584Publication of The Discoverie of Witchcraft (lists this demon)
1641Publication of the Lesser Key of Solomon (lists this demon)