I started to dabble with it before I got my FR-8X.
The USB to MIDI intface acceprs the MIDI signals from my MIDI keyboard, it captures the input to a track that routes it to another track where I have a virtual instrurment assigned and tgat channel is assigned via ASIO driver out to a channel to a specific channel on my FireWire mixer, out to the amp and speakers.
I found it quite complex to get it all working from the DAW side, but once set up, it was interesting. The thing was that costs involved for the purchase of Kontact and then other virtual accordions and virtual instruments can quickly reach from the hundreds to the thousands and the hardware needed go make it run perfectly needed to be above average if you started to get as complex as I was doing it.
It is surprisingly more cost effective to purchase a Roland V-Accirdion and install all the expansion sets and get more accordion sounds and flexibility over doing it via VST. Once properly set up, an FR-8X, for example can offer you several hundred accordion sounds that can be customized in to thousands of variations and then add the 15 separate drum sets and the several hundred more orchestral sounds and finally, being able to layer all of these possible combinations all from the keyboard, once set up, and you start to see that VST is not cost efficient and more studio onlythan anything else. Oh, you can use VST live, but the amount of preporatory work would be tremendous, at least 10 times the time and effort needed to use the V-Accordion and still never as flexible on the fly. Add a Roland BK-7m, use MIDI capture and integration and there is enough there to keep pretty much anyone busy for years... lol
Just my opinion.