I've heard of higher notes being easier on closed vowels, but is this something that should be payed attention to in writing or is it up to the performer to adjust their vowels?
The easiest answer is to add a compressor. This will bring up the quieter parts and bring down the louder parts. Be careful overdoing it, as it will make them less dynamic. You want to find the balance between level volume and decent dynamics.
Less easy answer is automation, where you adjust...
I'm afraid it probably is what it is at this point, but can you post the recording? It's more complicated than just using EQ, as the vocals will still be present at all frequency ranges