I've always felt that to be the case that Duke used vocalists as instrumental colors as much as obligatory additions to a working band.
I dig Hibbler, probably easily acquired the taste. . . . I don't know, he's just rich and deep and bold sounding. Like Harry Carney crossed with Hodges' tricks more than anything. Bird dug him as a person!
Joya Sherrill was the one I thought that was most a real vocalist (after Ivie and Jeffries), and still served as the instrument for Duke. (And what a BABE!)
And Alice too; I really want to get stuff by Alice outside of Duke and there are a few Ducal things with Alice I want still.