The problem is not filtering in vArranger or loopback.

It's simply that when you press a button on the arranger, it does what it has to do.

Ketron is better than other, because it sends a SYSEX every time you press a button.

So you can assign the PIANO button of the Audya to the PIANO button of vArranger, but when you press this button, it will select the piano section in vArranger, but also in the Audya.
It's not a problem.

But some buttons, like MENU or DISK, etc... it's better to not assign them, because you think you will control vArranger, but in fact you are removing some files in your Audya together