You are entirely free to put vowel diacritics when writing with the perso-arabic script and it's done in children's books. By that logic Vietnamese shouldn't be

You are entirely free to put vowel diacritics when writing with the perso-arabic script and it's done in children's books. By that logic Vietnamese shouldn't be written with the latin script since it's sino-tibetian Also, middle persian had a native abjad [1]. (As well as a derived alphabet [2]) 1: https://en.wikipedia.org/wiki/Pahlavi_scripts https://en.wikipedia.org/wiki/Pahlavi_scripts 2: https://en.wikipedia.org/wiki/Avestan_alphabet https://en.wikipedia.org/wiki/Avestan_alphabet