Depends on how the voice chat is implemented- if Vivox gives you access to the underlying audio buffer then you should be able to send that to FMOD and then spatialize it.
There was a discussion recently about voice chat with FMOD that is worth reading through for ideas on how to pass voice chat data to FMOD. There ended up being some issues with Unity’s OnAudioFilterRead callback that affected spatialization so that is definitely something to be aware of.