Vowels can be the bane of any choir leader. There is the discussion of which vowels as in old English, new English, different language vowels etc.
One thing I know for sure is that vowels must be round and forward to carry the sound.
This piece of music is an actual composition. These two young men have been taught and use some superb vowel production. Listen carefully the second time for the vowels. The first time is just too funny to concentrate.