Merlin is described as a prophet in the text. There are a number of episodes in which he loses his mind and lives in the wilderness like a wild animal. It is the first work to describe the Arthurian sorceress Morgan le Fay, as Morgen.
Geoffrey had written of Merlin in his two previous works, the Prophetiae Merlini, purporting to be a series of prophecies from the sage, and the Historia Regum Britanniae, which is the first work presenting a link between Merlin and King Arthur.
The Vita Merlini presents an account of Merlin much more faithful to the Welsh traditions about Myrddin Wyllt, the archetype behind Geoffrey's composite figure of Merlin. Whereas the Historia had Merlin associating with Arthur, his father Uther Pendragon, and his uncle Ambrosius in the 5th century, the Vita's timeframe is during the late 6th century, and includes references to various figures from that period, including Gwenddoleu and Taliesin. Geoffrey attempts to synchronize the Vita with his earlier work by having Merlin mention he had been with Arthur long before this.