Bel Canto Forum

In the course of listening to various recordings, I notice at about G5 or so, the vowels of sopranos become rather unintelligible. The aria "Glitter and be Gay" for instance contains a line: Until my maiden hand was gained... In listening to Gruberova's version, that gained becomes modified to a schwa of some sort. Is there any way to sing vowels that are intelligible up there? In Caro Nome, could a soprano modify closer to an open eh on the G#5 of "E fin l'ultimo?" I know a bit about formant theory and how pure vowels and intelligibility are determined by formant tracking. Is there any way to sing anything but a "uh" sound up there?