Just wanted to share this tip because I’ve seen the statement that it “can’t be done” pop up some times on the board 
This technique requires a VST vocoder, two sample tracks, and a send channel to work.
Normally a vocoder works by combining harmonic and dynamic material, such as a voice and some percussive material. The harmonic content meant to be vocoded is called the “carrier signal”. Most vocoders work with two input channels meaning that although the output may be stereo (due to some internal processing/synthesizer), the inputs are actually mono.
So what we want to do is to send two mono signals and combine them using the send channel. Start by adding the VST vocoder to a send track.
To add sound:
First, choose the harmonic channel and turn the panning all the way to the left (or right, depending on the channel the vocoder handles uses as carrier signal). Add a send device in the trackDSP, under META devices, and make it point to the track with the vocoder running.
Then choose the dynamic channel and turn the panning all the way to the right (or opposite side of the slider you just turned). Add a send device and make it point to the same track as before.
If you don’t hear a signal by now: select the send track (I am assuming you have some VST vocoder running here already), and adjust the settings. It is important that the carrier signal is assigned to the correct channel. If this is not possible, it may help to swap the panning on the two sample tracks.
Hope this is useful information 