I started receiving the dreaded "The following content is not available on this app" message yesterday on my YouTube Vanced. I fixed it by deleting YouTube Vanced, and downloading the following patch:
Warning : Must clean installation. You CAN'T update from 17.45.36 to this version because SIGNATURE has changed. root variant can update normally without uninstalling current installed app. Newer ...
github.com
Once it's downloaded, you must install it using a different APK manager (APK Mirror Installer in my case), meaning that Vanced Manager is not involved in this process. Not only did YouTube Vanced start working as usual, but it was updated to the newest YouTube layout as well.