After setting up GVI and perf tool the way it is described in the Halion PDF, I realized that it would be close to useless in regards to doing some actual work. Yes it sort of works, but really: 4 midi tracks per voice and moving the parts around for recording/playback. In real life that is a no-go for me. (I use Nuendo 3).
After sweating over it for half a day, here is the solution that works:
Download Midi Ox and LoopBe30. (Midi Ox is free for non-commercial use, and LoopBe30 is about 17 euro, but there is a fully working demo).
Install both. You probably already have VSL midirouter and MapleMidiport installed, and thats fine. (You need them both).
Set LoopBe30 to make 16 midi ports, and reboot.
Run Midi Ox. (We get back to that)
Your midi track in Nuendo (or whatever host) has your hardware input and VSL midirouter as output.
Perf. Tool has VSL midirouter as input and MapleMidiOut1 as output.
In Midi Ox you have to make a template that uses MapleMidi as input and routes it to all 16 LoopBe ports (these are called "Internal Midi"). Be aware, I am talking about 16 midi ports, not channels!)
On each of the vitual wires in MidiOX that connects to the outputs, there is a little box. Click on it and you see that you can mute unwanted midi channels. Set it up so every channel exept ch 1 is muted on out port 1.
And that every channel exept ch 2 is muted on out port 2. Etc. until port 16.
Now back to the host. Say you have a trumpet on track 1 set up to send to VSL perf tool. Now you have to set up a track to recieve from midi ox using the Internal 1 as input. Set the output of this track to GVI ch1 and switch on the little orange monitor speaker. Now if you are on your original trumpet track it should be playing correctly.
It may sound like a lot of work, BUT: The beauty is that you can make a setup that has all 16 of the passive monitor channels all set up, and in Nuendo you just use the "toggle" function to get them out of your way. You never have to look at them again, and after the setup, your work routines will be identical to what they were when your were just sending midi to an external GIGApc.
Sorry if this explanation is a bit rudimentary, but I dont have the time to make a manual "for dummies". But feel free to ask me about it. I can only say that is it worth it to figure the setup out. It really works: The workaround is hidden away, and you just have one midi track per voice, as you should.