If you’re not sure what makes a jazz singer then the following definition may help (supplied by bass player Ken Butler):
Vocal: Singers are whimsical creations of the all-powerful Jazz gods. They are placed in sessions to test musicians’ capacity for suffering. They are not of the Jazz world, but enter it surreptitiously. Example: a young woman is playing minor roles in college musical theater. One day a misguided campus newspaper critic describes her singing as “…jazzy.” Voila! A star is born! Quickly she learns “My Funny Valentine,” “Summertime,” and “Route 66.” Her training complete, she embarks on a campaign of musical terrorism. Musicians flee from the bandstand as she approaches. Those who must remain feel the full fury of the Jazz universe. The vocalist will try to seduce you and the rest of the audience by making eye contact, acknowledging your presence, even talking to you between tunes. DO NOT FALL INTO THIS TRAP! Look away, make your distaste obvious. Otherwise the other musicians will avoid you during their breaks. Incidentally, if you talk to a vocalist during a break, she will introduce you to her “manager”.