I didn't want to speak about the lengths because it seems to me sometimes it's impossible to do them right in a song unless the lyrics are perfectly composed that the long vowels coincide with the long tones etc. (if not, breaking the vowel lengths can actually be harmonically/aesthetically a better solution than to keep them as they are). Otherwise, (if it's not a song) I'm quite sensitive when it comes to vowel lengths, but I'm quite benevolent when it comes to singing (like, I like the Latin songs by Enya).