vMix supports VST3 plugins, but unfortunately it does not support passing MIDI events at all. Instead, the plugins on an audio input or an audio bus are just a linear chain of plugins without any MIDI input channels or output channels. Currently the only possibility to control VST plugins via MIDI events it to offload the audio processing from vMix to an external VST host like Cantabile (which has perfect MIDI event support) with the help of e.g. the NDI Tools VST plugin or the BlueCat's Connector VST plugin. As vMix already has MIDI support for control devices, it should also support the receiving and sending MIDI events to/from the VST plugins to allow one to remotely control those plugins, too.

A typical use-case is to use e.g. BlueCat's DP Meter Pro VST plugin inside vMix to send out the current volume to a TouchOSC-based volume meter. Another use-case is to send MIDI events into vMix to e.g. a BlueCat's Gain VST plugin in order to drain the volume before it hit subsequence VST plugins like the WT AutoMixer.