There are two types of vocal jazz ensemble.
One (call it the Lambert, Hendricks, and Ross-type) uses group singing as a means to deliver improvisation, solo artistry, and essentially a vocal cover of instrumental jazz. It’s a jazz combo, but voices.
The other (call it The Singers Unlimited-type) uses group singing as a means to explore harmony, expressively deliver text and melody, and a sense of ensemble sound. It’s a choir, but jazz.
When we say “vocal jazz”, you might think of one or the other of these things, but the truth is that most interpreters in the vocal jazz world fall somewhere on a spectrum between the two. LH&R, for example, placed no emphasis on exploring harmony or a sense of ensemble. TSU almost never features vocal improvisation, and its covers are more likely to be of rock songs with vocals than of instrumental songs.
Every other group falls somewhere in between. Manhattan Transfer bounces back and forth but is more comfortable, I’d say, in the TSU-type. The Real Group is even further on the TSU-type side, though they do have an improvisatory element based on real-time interaction among the members. New York Voices, to me, is the perfect marriage of the two types, with their singers able to draw seamlessly from both styles in a single song – sometimes in a single bar!
When you think about exploring vocal jazz, it’s essential to address, at least to yourself, that there are two vocal jazz ensemble types. Maybe that helps you select the right repertoire, or approach the ensemble, or just to get over your fear of this art form!