The name "tempo" doesn't mean anything - you're supposed to pick a name that best describes the soft key's effect, but it is not assigned to any specific parameter by itself. You basically have two options:
- find a device that sends MIDI clock that can be adjusted easily (
this is the ultimate standalone one, but many sequencers and controller keyboards can do it)
- find out how to control the Virus clock by SysEx and program a controller to do that. I'd like to be more helpful here, but the Virus TI SysEx documentation doesn't seem to be easily available (there's
this, but being from 1999 I'm not sure if it applies to the TI)
In any case, keep in mind that tempo applies to the whole synth, rather than a single patch.
As for your second question - I'm pretty sure it's impossible. The amplifier is controlled solely by the volume envelope, triggered by MIDI note - there's no "VCA level" parameter.