Make sure that as you transition through the vocal line, each successive vowel is LIFTED into the vowel space under your hard palette, not allowed to fall into the jaw.
To feel the difference, try singing (on one note) “we are” and notice the way it feels to LIFT out of the EE to a bright AH (for “are”). Now go EE and let the AH (for “are”) fall into the jaw space. Doesn’t sound that bad, but you’ll lose pitch with that approach, and ring fewer chords with the ‘darker’ vowel sound. Can you hear and feel that the ‘dropped’ vowel has had the top resonators chopped out of it?