Audio Bus Manager
Open up the Audio Bus Manager from the menu in the bottom right hand corner of the main vMix window.
Audio Bus Manager allows bus assignments for all inputs from a single window.
Click the squares within the grid to turn on or off a bus for a particular input.
Input Bus Mixer
Click the cog icon to open the Bus Mixer for a particular input.
Show inputs only of a particular type from the dropdown box at the top left of the window.