In this paper, a chapter of an Old Salentino medieval text is edited and analyzed, for the first time, as a source of linguistic information. The so-colled Grigoriu is an anonymous vernacular translation of the Tetrastichae Sententiae, a 4th century Greek poetic work by Gregory of Nazianzus. The Old Salentino translation, written in the Greek alphabet, survives in a unique manuscript, the codex Vaticano Greco 2252, copied in Terra d'Otranto around the 1330s. The Sententiae are here translated along with the commentary on the text by Nicola / Nilo Dossapatre, written in the 12th century. A linguistic analysis shows the importance of the data provided by the Vatican manuscript for the reconstruction of the historical grammar of the Salentino dialects. [ABSTRACT FROM AUTHOR]