tylermunns, that is a terribly narrow definition of what a Jazz singer is; and one that I could not disagree with more. Of the singers you mention, I may grant you Sinatra……maybe. However, Ellla, Vaughn not Jazz singers?! Ella Fitzgerald, with the possible exception of Eddie Henderson and today’s Kurt Elling was possibly the greatest vocal improviser ever. There have been very few vocalists who have had the vocal technique and, more importantly, the command of harmony and the general vocabulary of improvised Jazz to be able to scat in a way competitive with good instrumental Jazz improvisers. Surely, that alone puts her in the Jazz category. Of course, all this in her generational style which was rooted in Swing and, as you say, American Songbook standards. Btw, the vast majority of Jazz instrumentalist also used, and still use, the Great American Songbook as the primary source of their material. So…..
Abbey Lincoln was fabulous no doubt. Being a civil rights activist her later material reflects this. Her “protest songs” during the time that she was married to Roach were a reflection of the social climate of the time; just as was some of the instrumental Jazz of that time. However, her earlier recordings were centered on the American Songbook. Does this mean she wasn’t a Jazz singer then?