Your account (email address) is linked to the MAC address of the hdx. You would need to talk to vtuner (or Naim) to get it switched. So if you sell the hdx someone else can re register the MAC address again.
Just open a new account login with vTuner using the NDX2 MAC address and it will work independently of your HDX.
Of course it would be considerate to delete the old account from the HDX in case a new user wanted to create their own, although there is every chance that a new user would never need it.
You hardly ever need a vTuner account. If a station isn’t listed in the Naim app, and Naim are unable to ask them to add it, you can add the URL yourself by logging into vTuner. You will then find in Added Stations in the iRadio input.