In order to achieve what you want, each button on the Vos needs to have a “toggle” mode, so that it can go between two states. A cursory look at the Vos manual and it does not look like it has a dedicated “toggle” feature, but it does have “Long Press” to access a second layer or “Step By Step” commands, where states changes with consecutive presses. You’ll have to experiment with those to see if you can achieve the functionality you want. Perhaps send Vos an email and ask them how they recommend you achieve what you want.
Any time you send the H9 a PC (Program Change) message, it will automatically load the preset as activated; unless you save a preset on the H9 in a bypassed state on purpose, the default is loading with Active on. In your example, you’d want Button 6 to send PC#5. You could then use Long Press to send a CC message to bypass or use the Step By Step feature to turn the H9 off or on with consecutive presses without sending another PC message.
If you’re using H9 Control and you go to the PEDAL tab, click on MIDI SETTINGS, then ASSIGN MIDI CC MESSAGES. There you will see a list of parameters than can be assigned their own CC#. Actions like Bypass, Activate, and Toggle Between Active and Bypass are assignable.