VMI (Virtual MIDI Input) configures a plugin to appear to the rest of the rack as a virtual MIDI input port. This is a way to utilize MIDI events produced by plugins. If you press VMI for a plugin, a new MIDI port is created for all other plugins in the rack. This new port is disabled by default but can be configured exactly like any other MIDI port. Be careful not to enable a plugin to receive its own MIDI output if there is any chance it will simply echo that output. This would set up a feedback loop of MIDI events.