By convention, MIDI control change number 66 is for Sostenuto. But Sostenuto is not implemented on the Virus TI, as Access Music Support have confirmed: see http://virus.info/forum/index.php?page=Thread&postID=13996. Configuring the Virus to interpret the hold pedal (foot switch) as indicating CC#66 Sostenuto has no effect.
If you are wondering what Sostenuto is supposed to do, it emulates the behaviour of a grand piano's middle pedal. With Sustain, note release is blocked for all notes played while the hold pedal is on. With Sostenuto, all notes that were already on when the hold pedal is pressed will have their releases blocked while all notes played while the hold pedal is on will release normally. A typical use would be to play a chord and, while the chord is held down, press Sostenuto, then, while Sostenuto is on, take the hands off the chord and play some more notes. This is really good for sustaining a chord without having to hold it down while playing a melodic run over the top of it.
If Sostenuto were to be implemented on the Virus, I think I would use it more than Sustain. With many of the more full-bodies patches, Sustain very quickly becomes overwhelming. Sostenuto would get round that problem by allowing the musician to decide which particular notes are to be sustained.
Of course I don't know how feasible it would be to implement Sostenuto on the Virus. I can see that it might take more processing power. With Sustain, the Virus only has to process Note On and Note Off differently once the pedal is pressed. With Sostenuto, while the pedal is not pressed, the Virus would always need to keep a record of which notes are currently on, in case the Sostenuto pedal is pressed. Maybe this information is already available for other purposes, I don't know.
Simon