This depends entirely on what frequency (pitch), and to a certain extent amplitude (loudness), is being sung. Vowels are defined and perceived through their unique formants or resonances of the vocal tract. For a very low pitch, such as that sung by a bass, it may require two or even three harmonic overtones for the vowel to become discernible because the formant sits a couple of octaves above the fundamental frequency being sung. However, for very high pitches, such as those sung in the upper female ranges, this fundamental pitch may exceed the location of the formant(s) for a specific vowel. In this case no amount of harmonic overtones can produce the necessary vowel. The laws of acoustics force it to modify towards another vowel containing higher formant locations.