MIDI 126 is "Mono On"... seems like we can only turn it on.  I think we would need to hit CC #127 "Mono Off" to turn mono-mode off again. We'd just have two different destinations, turning one or the other on.

MIDI 68 is "Legato" and the description is "Forces mono playback". We could maybe turn that one on and off.

I haven't tried these.  Good luck!

@mikeh11 Thank for your reply. There are a few good tutorials about VAST on youtube and there is a pc3 on which the midi 126 with 127 value turns on mono and with the value of 0 turns it off. An it is mentioned that you can do it with midi127 too... There are a lot of similarities between pc3 and forte, and of course the main thing is almost the same, but for some reason I have to use two switches to be able to turn on and off the mono. Anyway, I will try the legato thing.

Cheers! 

I tried the mono-switch both ways; on a PC3LE6 running 2.27. Both methods worked.

One way: turn on "Mono On" and turn on "Mono Off"

  Type: toggle
  DestType: ctrl
  On Value: 127
  Off Value: 127
  On Dest: Mono On
  Off Dest: Mono Off

Was able to switch from chords to single-notes by toggling the button on and off.

Second way: turn on and off "Legato"

  Type: toggle
  DestType: ctrl
  On Value: 127
  Off Value: 0
  On Dest: Legato (#68)
  Off Dest: Legato

Same as before, switch from chords to single-notes by toggling the second button on and off.