What’s exactly the “polyphonic aftertouch” effect you get when using the volume column with a vst instrument? Is it documented somewhere? I didn’t find any reference in the user manual.
What do you mean? It is up to whoever programmed the VSTi how they use Aftertouch, if at all, to affect the sound.
Couldn’t find it in the Tutorial pages but there is a note about it for the 2.0 release, when it was added.
Well, I was working on a bassline and changin instruments to find a starting point for a sound. At some point I tried the vst Cakewalk Z3ta and it sounded strange on the notes that had the volume set in the volume column. So trying to insert a volume next to these notes I noticed the message in the bottom window of Renoise that says: set volume velocity or polyphonic aftertouch for MIDI/VST instruments. So I thought was this to cause the strangeness in the sound, but there’s seems to be nothing in the user manual.
Well to find out what the effect is I suggest you read the Z3ta manual 
Firstly. If it’s next to the Note is should be initial Velocity on the Note.
If it is in a Note Column but not next to a Note it will be Key Pressure (Aftertouch) for the last Note played in that Note Column.
Both can be linked to just about anything. Velocity will often just be Volume, maybe Filter Cutoff or Resonance too. Aftertouch similar but more often trying to change the presence without changing the volume too much, so may be Filter Cutoff/Resonance again, could be Vibrato Depth, some kind of Exciter or Chorus or any part of the synth patch that you might want to change while notes are playing to give feeling to the notes.
It is simply sending another velocity value on an already playing note (not initializing a new note) but how this is interpreted differs by various plugins.
Monophonic aftertouch can be for certain controlled using the midi C3 command (channel pressure)
http://tutorials.ren…s#MIDI_Commands
A good find though, i have added the missing info on the command effects page.
Would be really nice to use channel aftertouch as polyphonic aftertouch - much easier to edit afterwards and could be used to control volume of xrni’s.
Extra checkbox in midi settings would be enough for this. “Use channel aftertouch as polyphonic aftertouch on active column”
I should probably do a feature request 
Channel Aftertouch by definition is not Polyphonic!