The vowel exchange "a" to "o"

[Mauro]

Unlike the Italian examples that exchange at the end of a word for grammatical reasons

Not to be picky but, in addition to changing at the end for grammatical reason, notice many Italian words also do change midword. In the Italian case (but I guess it holds for very many languages) it's just because a consonant followed by a wovel is a basic syllable, so if syllable 'po' exists, also 'pa', 'pi', 'pe', and 'pu' will probably exist, and if a word contains 'po' there is some probability the same word with 'po' replaced by 'pa', 'pi', 'pe, or 'pu' will exist too. And indeed: pozza [puddle], pazza [madwoman], pizza [pizza], pezza [piece of cloth], puzza [stench], where the vowels are exchanged midword.

There's nothing strange in this and, importantly and unfortunately, there is little to glean from noticing that (above the general morphology of Italian syllables), because puddle, madwoman, pizza, piece of cloth and stench have little in common, except differing by just one vowel midword in Italian. The Voynich could behave similarly, so having 'choin' and 'chain' will not be much of help in understanding what 'choin' and 'chain' are. At the opposite, it might manage plurals by changing a character midword instead than at the end as we're accustomed to, so 'one choin' but 'two chain', and this would be very much interesting instead, who knows (and who knows what else). Not that I endorse any of these two ideas, at all, it's just to point out there are always a lot of possibilities to explain why things are made in a certain way, and discriminating between them in the VMS case is difficult. Cautioning against this was the overall sense of my post about Italian, beyond giving an example as requested.