Good info! Just to add, I had to make the same discovery with Sonivox VocalizerPro. It is also an FX plugin that you insert into an audio track, but you still need to add a MIDI track to "play the effect" through its FX MIDI input.
Additionally, I've learned that with Sonar's new patch points you can route live audio output from any soft synth to the audio track with the vocal FX plugin. You don't need a pre-recorded audio clip or an external audio source, such as a microphone or instrument. Just configure any soft synth to "Output = [New Patch Point]". Then set the audio track with the vocal FX to "Input = Patch Point 1". The sound from the soft synth is now routing to the audio track instead of the default Master output, and is then processed by the vocal FX on the selected audio track.