I'm from England, but sing in a barbershop choir. The only accent I can sing well with is a hybrid of British and American accent. European vocal music is not sung in an American accent, but with those rich, tall vowel sounds, it's just a thought......

I follow what has been the standard practice in diction for vocal artists-you sing the music in the accent of the nationality of the poet or lyricist if it is a song,or in the case of an operatic or musical theatre character, the accent of the charachter, not what is easiest.

That being said, if you are singing in an ensemble, you must, of course match your vowels exactly to the other singers.