I was experiencing an issue with a fresh install on a new machine where opening the Decoders tab in the vMix settings would completely lock up the application. vMix would become unclosable, and the entire system would have to be rebooted.

The issue was bad drivers for the "Realtek USB Audio Front" and "Realtek USB Audio Rear" devices. I am not sure where the drivers were installed from -- perhaps Windows update.

The fix was to download the drivers from the manufacturer's website.