The more you open your mouth, the more air you can move, which often allows vocalists to support higher notes better. If your vocalist is straining to hit a note, changing the vowel shapes may make a difference.
And I have worked with one vocalist who has asked that I watch my vowels in her higher parts.
So it is something to consider.
But I generally wouldn't give it too much mind. Write the song as you want and then if the vocalist just can't sing it, adjust what you need to.