Sorry, it’s hard to help when I’m unfamiliar with so many parts of the system. If the vibrandoneon has a bellow, what is the pipe used for?
In any case, if you use a MIDI instrument that supports breath control, you can just plug all MIDI instruments in to your ipad, assuming that the ipad accepts USB MIDI devices (and I would guess it does). If you have only one MIDI port, you use a MIDI hub. That’s it.
Usually, everything transmits on Channel 1. You don’t have velocity sensing so I assume the notes come in with a fixed velocity. So just plug in a USB breath controller or a USB expression pedal and you’re done. You might be assuming that it’s harder than it is.
The MIDI instrument needs to support breath control or expression pedal. You would need to check the docs on the instrument you plan to use. If it doesn’t, pick something that does or look up “how to map MIDI controllers on an ipad” to find out what’s possible. As I said, almost everything supports volume (CC 7), so you can always try mapping to that.
To make MIDI work, you need the controllers but you also need hardware/software that makes a sound. There isn’t a universal answer for how to make something work; you have to choose all your components. This is true whether you use one controller or more than one. What kind of sound do you want? If you view an Audio Modeling demo and think “That’s what I want!” then your controller is not going to be a good match. If you use Joe’s Discount MIDI, then your controller is fine, but you might need some CC mapping capability.