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.

Filter Inputs

Show inputs only of a particular type from the dropdown box at the top left of the window.