I’ve been a Jazz fan for some time now, but only recently have taken interest in vocals, particularly since discovering Cathy Ann Wells, a Canadian Blues and Jazz singer in Kelowna, B.C. (The mp3 attached to that first link is her singing Blues, remarkable and powerful to be sure, but do yourself a favour and find her mostly Jazz live album where her dynamic range is all laid out for you to fully appreciate.)
I don’t often listen closely to vocals. As an amateur musician, I tend to listen to the instruments and rhythms. I’ve been listening to vocals lately though as another instrument, not following the words as much as the modulation, pitch and timbre. Jazz vocals tend to suit this type of listening well.