This article, intended for non-specialist Vietnamese readers, begins with a typographical curiosity in the romanized spelling of the famous Dictionarium of Alexandre de Rhodes (1651): the modified ꞗ, indicating the bilabial spirant transcribed [β] by linguists. In the 17th century, this phoneme was distinct from both b [ɓ] and v [v], but neither the modified ꞗ nor its pronunciation survives in modern standard Vietnamese: words written with ꞗ in the Dictionarium are currently written and pronounced with v.The spelling of vernacular plant-names in post-Dictionarium botanical works is examined for clues to the date of the merger between β and v. Although examples of the erroneous use of ꞗ for v in de Loureiro 1790 may be an indication of merger, the replacement of ꞗ by b in later works by French botanists is a strictly typographical error with no basis in phonology, because the phoneme β merged only with v, never with b [ɓ]. [This is a translation of Haudricourt’s original French typescript, prepared by Nguyen Phu Phong and published in 2005 as: Les deux b du Dictionarium d’A. de Rhodes, Cahiers d’Etudes Vietnamiennes 18, 65–68.]