One of the issues we have, which shows itself more during song work where we are changing from one vowel to the next in rapid succession over a range of melodic notes than in exercises, is that each vowel has a different 'tendency' or 'drive.'
Some vowels tend more to the head and some tend more to the chest. It's a simple rule though: The narrower vowels tend to drive mopre head, the wider vowels drive more chest.
There is a logic to it: because the chest voice is made in the throat, and the throat is adjusting size to form the different vowels, it follows that the larger the throat, the more chest in the sound. You know by now that the |ᴜ| vowel is the smallest throat and the |ɑ| is the largest.
Due to the underlying throat working in the same way, the front vowels exhibit a similar effect, but front vowels also have an additioonal advantage for us: Because we are amplifying the key harmonics in the front of the mouth, it is possible to sing higher/with a harder tone than it is with round vowels. See the article below.
Initially this causes problems for us as we sing, but the more we work with vowels and melody the more familiar we become until we learn how to overcome the negative aspects of this effect and learn instead to leverage it for creation of melismas and other effects, as well as for styling of our sound.
In the next chapter we deal with the cords and the concept of 'press' or larynx stabilisation, which is what we have to use to counter (as well as eventually leverage) the varied drive of the different vowels.