Okay here's what I do..
The majority of VSTi's don't seem to care about wether you assign a channel or not, I only came accross one that did but I forget what it was
For this example I'll use Revitar, as that's the latest VSTi i have
Under the General page, I load the VSTi into the plugin area. Then I go to the instruments page, create a new instrument, and in the plugin section there I then select the VSTi from the drop down list.
Done, and it works..you can then of course edit the settings for the VSTi but that's a different kettle of fish.
One thing that springs to mind though, if you are using a drum synth (or drum vsti that uses samples, like sr202) I noticed the majority of them you'll need to have the octave set to 3 to hear anything when playing.
Hope this helps, I know it's not written up well, i'm at work and my music pc is at home