In his book, “Singing of the Future”, Ffrangçon-Davies had this to say on the importance of vowel purity in learning to sing:

Voices are “produced” and “placed” in such wise that pupils are trained to “vocalise” (to use technical jargon) the words, i.e., they are taught to make a sound which is indeed something like but is not the word in its purity.

“Tone” or sound is what the average student seeks [these days] from the beginning and not verbal purity.

Hence the monotony of modern singing. When one hears an average singer in one role, one hears him in all.

…All pronouncing which cramps the throat is wrong. There is no part of the vocal range where trickery (so called vocalizing) is necessary; no altering of the character of the word is admissible; else you might as well sing in Choctaw and expect people to get the full effect of English!

Ffrangcon-Davies, David Thomas. The singing of the future. J. Lane, 1907.