- -Job-: Unforuntately, all we have is Korean documents saying King Sejong's protect was finished in 1443-44 and then King Sejong's book from 1446. We have nothing left of his actual research and earlier attempts to come up with a script, so we can only speculate.
Now I would suggest taking any Korean text from the period, preferably one written completely in hangeul, then rewrite it so the jamo (single components of hangeul syllable blocks) are written sequentially like an alphabet, and decompose all vowels into their constituent components too. In this way, eg. the world hangeul itself, written 한글, would be written ㅎㅣㆍㄴㄱㅡㄹ. Then run statistics on thus written Korean text and see if it matches Voynichese in any way.
Edit: And even if the script turns out to be Korean, the language itself itself might not necessarily be. Even hangeul had several now obsolete jamo that were designed to write down sounds found in contemporary Chinese but not Korean, so if this is indeed a preliminary pre-hangeul script of King Sejong, it might write down Chinese too, so the manuscript might contain Korean, Chinese, or even both which was common to write in Korea at the time, and which could explain the seeming two "languages" of the manuscript.
Now I would suggest taking any Korean text from the period, preferably one written completely in hangeul, then rewrite it so the jamo (single components of hangeul syllable blocks) are written sequentially like an alphabet, and decompose all vowels into their constituent components too. In this way, eg. the world hangeul itself, written 한글, would be written ㅎㅣㆍㄴㄱㅡㄹ. Then run statistics on thus written Korean text and see if it matches Voynichese in any way.
Edit: And even if the script turns out to be Korean, the language itself itself might not necessarily be. Even hangeul had several now obsolete jamo that were designed to write down sounds found in contemporary Chinese but not Korean, so if this is indeed a preliminary pre-hangeul script of King Sejong, it might write down Chinese too, so the manuscript might contain Korean, Chinese, or even both which was common to write in Korea at the time, and which could explain the seeming two "languages" of the manuscript.