Not sure if this helps anybody, still:
I use VoiceAttack but all the voice comms where done with/through UHF, or VHF, depending on the joystick button I assigned on my HOTAS (Thrustmaster Cougar) i.e. in VA Joystick button 20, or 21.
I had in mind to use both Comms buttons - UHF and VHF - on the HOTAS stick for specific voice commands through the required channel (UHF, or VHF). So, voice comms for the Tower, Tanker, AWACS etc. are done with UHF and internal flight communication (voice comms for the Wingman and Element) are broadcasted with VHF.
I have made a setup in VoiceAttack and it works for me to use the HOTAS stick and choose the required comms channel (UHF or VHF) to give the specific voice command.
Note: with this setup in VA for the HOTAS stick on both UHF and VHF all VA voice commands are broadcasted. With this setup it only makes it a bit more realistic to choose the specific broadcast channel to give a voice command.