Make sure the NDI camera you are purchasing is compatible based on vMix's hardware compatibility list.

Then, presuming your network switch is connected to a local area network (e.g. a router that is providing internet service), you'll likely need to follow the instructions that come with the camera and temporarily change the IP settings of your computer to match the default subnet settings that the camera uses. That will enable your computer and camera to be on the same subnetwork and you'll be able to then log into the camera's web interface and assign a static IP address for the camera that is within the range of your Local Area Network. After you've done that, you'll change your computer's IP settings back to what they were initially.

Once you've done all of that, you should be able to add the camera as an NDI input from the list of input type choices in vMix.

The camera should come with instructions that walk you through the second paragraph above. You might also reach out to the camera manufacturer for support if you need it.