Gd day ,
I would like to know if anyone has previously used the sidechain
function on the top right corner of the virus ti2 plugin in logic pro 9 ?
Id like to know if it can be used by routing signals internally via
Logic and is ther a way to adjust its senstivity/amount of ducking.
Thanks 
For the VC plugin, sidechaining means sending audio to the Virus for processing, such as vocoder, atomizer (rhythmic chopping up), and using the effects in the effects section. There is also one option that act like you describe called input follower which when used replaces the filter envelope source with the RMS amplitude of the input source, where you can control sensitivity with the filter envelope controls, and using the modulation matrix you can change the sound by selecting a destination to be controlled by it.
But you can achieve what you are describing just by adding a compressor plugin on one of the VC plugin's output tracks, and sidechaining it instead.
I know I didn't answer your question directly but I hope i still helped somewhat.
