All 3 are excellent at vocals. They sit on a continuum of IEMs that render vocals exceptionally well. The difference is how vocals are presented, and that comes down to a matter of taste and choice of music.
In Butastur, vocals are balanced more evenly with the rest of the instruments, whereas they are lusher, more forward and more refined in the Volt and Aurora.
Between Volt and Aurora, Aurora has more bass, particularly sub bass, although it’s very well controlled and you won’t hear it if the music doesn’t call for it. When it kicks in however, it can neutralise the vocals a bit.
Volt has the warmer, richer, more forward presentation, particularly in the lower mids and if you’re looking for
vocal performance above all else, it’s still hard to beat and I’m not surprised it’s still
getting rave reviews as recently as 2 weeks ago.