according to a Vatican curator:
"The Vatican Hill takes it name from the Latin word Vaticanus, a vaticiniis ferendis, in allusion to the oracles, or Vaticinia, which were anciently delivered here."
Sources: Compendious Description of the Museums of Ancient Sculpture, Greek and Roman, in the Vatican Palace, by Cav. H. J. Massi, First Curator of the Vatican Museums and Galleries, Paleographer and Professor of the Italian and French Languages, Rome, Third Edition, 1889, Title page, page 7.
Compendious Description of the Museums of Ancient Sculpture, Greek and Roman, in the Vatican Palace, by Cav. H. J. Massi, First Curator of the Vatican Museums and Galleries, Paleographer and Professor of Languages, Rome, Sixth Edition, 1901, Title page, page 5.
"Divining Hill" seems appropriate.
Note that the same word used for serpent is also used for enchanter, and that divination is forbidden by God.
But it does help explain the architecture.