Unfortunately tap tempo is not an assignable Switch-3 target for the VoiceLive Rack. The "Tap" source in the Delay effect is referring to the "Tap" touch button on the front of the Rack itself. Apologies for the confusion
Well, here I am, ridiculously, 10 months later, still trying to figure out a way to tap in a tempo live while not having the VoiveLive Rack right next to me or needing a free hand to push the physical tap tempo button. I'm shocked that tap tempo isn't assignable to the Switch 3, so I'm trying to use a Line 6 HDX Pod Pro X, which allows me to tap a tempo in via footswicth, as a tempo source for the Voicelive Rack. I've set the midi channels on both devices to 1, I have set the Delay source on the VoiceLive Rack to midi, and still... nothing. Has anyone found a way to change the Tap Tempo dynamically on the VoiceLive Rack? I think what I'm trying to accomplish here is super basic, and I'm getting frustrated as I hit brick wall after brick wall.