Why do I get a black screen when using Vcast and Youtube/Amazon prime/Netflix?

Refer to this FAQ.

Why is there no audio sharing from vCastSender for Android while screen-mirroring to ViewBoard?

This is due to a privacy policy by Google – Android phone/tablet. Google only allows apps that are pre-installed on your phone/tablet to capture audio and prevents third-party apps from recording audio in order to avoid user privacy invasion. Therefore, third-party apps cannot access the phone/tablet for audio output.