The vowel exchange "a" to "o"

[Petrasti]

that's right, in Romance languages as well as in Spanish this exchange exists, but several words in a grammatical unit bend such as
"la chica bonita" and "el chico bonito"
I have not yet been able to find this phenomenon in Voynich. It seems that the a and o can be exchanged flexibly without a ‘rule’. Personally, I can't imagine that the accumulation involves ‘different’ words.
Why do I think this is important? Well, we are looking for systems in the manuscript and trying to understand whether it is a language cryptology or nonsense.
Perhaps this can also exclude systems or languages that cannot find a sufficient explanation. But it's just a question of how you evaluate that