The variable configuration of the vocal cavities and the habits acquired by singers makes it necessary that the teacher should regulate vocal exercises according to the necessities of each person. All voices cannot be formed on one mould; a different treatment will be required to modify exaggerated peculiarities. By learning the effect of the action of these parts of the mouth and throat on the quality of tone, a tangible means of correction is gained. If a voice by nature of wrong habit is sharp or piercing, the practice of the vowel sounds o, or, ou, with the mouth dilated as much as possible in the interior, forming a tunnel-shape of which the back part of the mouth is the largest, will render the tones rounder and richer in character.
The opposite quality of dullness or headiness can be modified by using the open Italian a in vocalization, the mouth being much opened transversely, the large part of the tunnel in this case being the front. Other vowels suit other cases. It is for the teacher to discover what vowel is most favorable to the production of an agreeable timbre in each individual voice. The teacher should be able to imitate defective timbres and to explain their cause; and also to give examples of artistic timbres, until there is awakened in the pupil an appreciation of the beautiful in tones. Then all difficulties will be easily overcome.
Artistic Singing by Sabrina H. Dow, 31-32.
This is a very different approach than is used by many voice teachers today, who don't quite grasp the utility of working with pure vowels, the emphasis having shifted away from what is heard to what is seen, especially as the presence of voice analysis technology in the studio is concerned. As well, many work in a mechanical fashion, with an eye towards getting the muscles of the larynx to move according to pre-determined plan, registration being their main focus, with vowels being something of an afterthought.
My observation: once the student is busy looking, the ears stop listening. This is why I don't like having students read from music during their lessons. Nine times out of ten, their audio-vocal control suffers. Yes, professional choristers must master this skill, but even so, the best will tell you having the score memorized makes for a very different experience. Of course, the same thing goes for the conductor who is standing in front of you!
In accordance with this matter of listening, Dow emphasizes the ability of the teacher to demonstrate, which, it should be noted, gives the student an inordinate amount of information. Of course, the real trick is the student's ability to listen to technique, rather than blindly copy what is heard. As those who study with me often hear: "That was great! Now do it with your voice, not mine!
Dow is right about difficulties being overcome. If the student can't discern the difference between what the teacher does (and it better be good) and what they feel and hear (feeling is a vestibular function of the ear), there is no going forward.