If you are already bought VOX Radio via AppStore version you can restore it in standalone version.
Please try to uninstall VOX by AppCleaner, which you can download from the link below:
Once it's done, please launch AppStore, locate Vox under your Purchases tab, restore the radio and install Vox from there.
Then, download VOX by clicking on "Direct download" on this page:
Launch the .dmg file and drag the app's icon onto the Applications icon. You will be prompt about replacing the VOX app, click "Replace".