As I've been working with some singers who are at the beginning of their development as well as those at the beginning their professional careers, much of my attention has been focused upon vowels in singing. One basic truth learned early in my development rings true: Vowels in the unamplified singing voice which are borne of different timbres from one another become unintelligible. Likewise, vowels of a similar timbre compliment one another and pronunciation becomes clear. They have to be like magnets fitted together; each free from impedance and similar in timbre to the first. That, to me, is a "pure vowel." A singer must become a master matchmaker for their sung vowels, first working from what their ear knows as the correct spoken vowel, having good advice for correction when necessary. Being diligent with a younger singer (and also helpful to established professionals!) to find these pure vowels from the beginning are tantamount to setting them up well for the rest of their singing lives.
Particularly in the operatic genre, singing lives primarily on the aforementioned qualities to project a musical and textual thought into a large space. Some research into the old singing methods reveal that clarity of both text and tone, and freedom of expression were the goals of what some may call the "bel canto" tradition, as well as having a base method to present the text and music artfully, appropriately and with individual expression. More on this later....