Wikipedia
Vassago (also Vasago, Usagoo) is the third Goetic demon described in the Lesser Key of Solomon (including Thomas Rudd's variant) as a prince "of a good nature" (despite being a demon) and of the "same nature as Agares". He rules twenty-six legions of spirits, and is summoned to tell magicians of past and future events, and locate lost objects. He is one of the few spirits found in the Lesser Key of Solomon but not in Johann Weyer's Pseudomonarchia Daemonum. Vassago is mentioned in the Book of the Office of Spirits as Usagoo, appearing as an angel, "just and true in all his doings," with the powers of inciting the love of women and revealing hidden treasures, in addition to ruling twenty spirits. Sloane MS 3824 mentions Vassago in invocations to summon spirits that guard treasure, and in the "Experiment of Vassago" and the "Experiment of Agares," both intended to capture the named spirits in crystals. According to Rudd, Vassago is opposed by the Shemhamphorasch angel Sitael.
Murderer and vampire-themed cult leader Rod Ferrell used "Vassago" as a "vampire name" and burned a V representing this alias onto his victims.