With respect to Vietnamese, the alphabetic writing system with (lots of) diacritics is modern.
There was no alphabetic writing of it in the 15th century.
An alphabetic writing system for Thai arose in the 15th century, according to tradition.
This only indicated tones in specific cases.
In general, one can safely assume that any 15th century rendition of an East Asian language would have to be either a rendition of sounds, or a transliteration of a much larger alphabet of chinese-like characters. If it ever happened.
I am not aware of any original examples. They do arise as a result of all the later missionary contacts.
Indication of tones in writing is not at all something that was usually done, at least before modern romanisations.
Many of the monosyllabic Asian languages have adopted loan words from Indic languages (Sanskrit, Pali) as a result of the spread of Buddhism. Such words are mostly polysyllabic. I am not sure about the timing of this though.
There was no alphabetic writing of it in the 15th century.
An alphabetic writing system for Thai arose in the 15th century, according to tradition.
This only indicated tones in specific cases.
In general, one can safely assume that any 15th century rendition of an East Asian language would have to be either a rendition of sounds, or a transliteration of a much larger alphabet of chinese-like characters. If it ever happened.
I am not aware of any original examples. They do arise as a result of all the later missionary contacts.
Indication of tones in writing is not at all something that was usually done, at least before modern romanisations.
Many of the monosyllabic Asian languages have adopted loan words from Indic languages (Sanskrit, Pali) as a result of the spread of Buddhism. Such words are mostly polysyllabic. I am not sure about the timing of this though.