Miracast is useful to show your videos from a storage device to another one or to display internet videos on your TV. But it's a low quality protocol.

You can easily access your stored video files with your network or by connecting your device to your computer to copy the files. For internet videos you can simply use Web Browser inside vMix to play any video. So you have the best quality available.

If you really want to use Miracast with vMix, get a miracast dongle and plug it to a capture card.