1. First add the background to vMix as a separate input.
2. Click the i button under the input to open up the Input Settings
3. Under the MultiView tab select the camera from the first dropdown box.

You will now have combined the camera and the background into a single input.
You can go back to the input settings for the camera and use the Colour Key tab to configure the chroma key settings.

You can then select that combined input within another input MultiView setup.
(vMix supports up to two levels of MultiView in this way)

Regards,

Martin
vMix