vMix automatically supports alpha channel when it is detected on an NDI source.
If the source provides the video as "pre-multiplied alpha", be sure to turn on the checkbox in the Colour Adjust tab in Input Settings
When using the Output options with NDI, these will by default send standard "YUV" video without alpha channel.
To enable Alpha Channel on output follow the steps below:
1. Open vMix Settings and go to the Outputs / NDI tab
2. Click the cog icon next to the Output you wish to enable Alpha Channel for.
3. Under the Alpha Channel setting, select Straight or Premultiplied depending on the receiving application requirements.