My understanding is that the NDI version of desktop capture replaced the older version. You can't use the older version with newer releases of vMix.

Here's a slightly obtuse alternative - you can use a third-party desktop capture application, like DeskCamera.

These sorts of tools make the desktop appear like an RTSP stream to your network. they essentially make the desktop capture look like a surveillance camera. The original purpose is to allow a cash register desktop to be recorded to a large NVR, along with all the in-store cameras.

Still cheaper than a hardware-based approach to capturing the desktop.