I have been curious but unable to find the answer. Are you able to pair the Vanguard to a garmin computer that is paired with Varia and hear the car alerts on the vanguard? I can see this being very useful with or without listening to music.
In my experience you just pair your headphones with your phone and run the varia app while riding in order to get the audio alerts in your ears. The varia will happily connect to the app and your head unit at the same time.
I assume the vanguard can connect to a phone via Bluetooth? So should it work.