The old Italian and German masters, whose schools produced such excellent singers of both sexes, show by their writings that their ﬁrst endeavour was to teach their pupils how to form a beautiful tone and to render their voices flexible and of a telling quality. There was no question of exercises in a large compass, nor of developing a powerful tone on one vowel, as is now the fashion. In the elementary instruction—that is to say, in the solfeggio—our ancestors began by teaching how to establish and sustain the voice. They next proceeded to exercises in a small compass, such as are provided by ornaments (appoggiaturas, turns, mordents, etc.), and then let their pupils practice on the ﬁve elementary vowels and without accompaniment. We have to proceed in the same way if we wish our pupils to acquire not only a powerful, but also a beautiful and expressive tone, a ﬂexible voice, and good pronunciation.
Stockhausen, Julius. Extract from "A Method of Singing," The Voice, April 1888: page 58.
Stockhausen was a fellow student of Jenny Lind in the studio of Manuel García and went on to become one of the great Lieder singers of his generation as well as García's representative in Germany. In the passage above, he illustrates the simple methodology of the old Italian school which began with the formation of a beautiful tone, then progressed to the practice of the five Italian vowels without accompaniment. I can't tell you how many students would rather skip this stage of development. However, there is no getting around it. Either you master your vowels, or you don't. And if you don't, the voice never really improves.