vMix is a robust software-based live video switcher, titler, and streaming app, but it only has a rudimentary remote control interface. This app fixes that by turning any iOS device into a tally light for talent, a tally light for camera operators, or a full remote-control interface for the vMix system. Everything that can be done through the API can be done from your device.
Currently, the only limitations to this app are that it cannot display a live preview for the inputs. There is a workaround, though, that will give you thumbnail images for your inputs. Instructions for the workaround are in the app.