206joey I've got the same setup, and the same concern. I'm always worried I'm going to accidently leave the lead vocal stem on after a rehearsal at a gig 🙂 Not a major issue as I only have it routing to our IEM mix and not FoH, but it will still be confusing for the band.
As @jaytrax mentioned above, in ST3, the only option is to disable the routing assignment for that track in Settings -> Audio Routing -> Track 8 Output and set it to Disabled
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That's probably not going to work well based on the scenario you described where you are quite literally wanting to be able to turn it on and off during playback.
I also had the concern that because my X32 mixer presents 32 mono and 16 stereo channels tot he iPad, if I'm changing from disabled back to the Channel I've setup on my X32 for the vocal guide (in my case, Channel 31), I've got to make sure I remember the correct channel each time. So a global Mute On/Off would be much neater and safer.
I raised this topic with Peter for ST4 in the following thread and he advised that we will get the ability to Mute at a global level - https://support.stagetraxx.com/d/3331-global-track-mute-as-opposed-to-disable