You might be interested in the article, "The origin of the peculiarities of the Vietnamese alphabet", https://halshs.archives-ouvertes.fr/halshs-00918824/docume

You might be interested in the article, "The origin of the peculiarities of the Vietnamese alphabet", https://halshs.archives-ouvertes.fr/halshs-00918824/document https://halshs.archives-ouvertes.fr/halshs-00918824/document The short version of it is, The missionaries who adapted the Latin alphabet to Vietnamese were Portuguese, Italian and French. The resulting spelling inherited some peculiarities from the spelling systems of Romance languages. For example, on the phonology of 'ch', Among prepalatals, the unvoiced stop CH is taken from the Portuguese and the Spanish, which themselves borrowed this notation from Old French, where it had been created to transcribe a new sound, not found in Latin.