What you are trying to do is much more complicated that what I was first thinking. Volcas can be sometimes tricky to synchronize with everything else, and still, you have to deal with Sonic-Pi to synchronize. I would suggest you to try something different to achieve what you want.
For instance, you could benefit from using the live_audio type of loop, as suggested by Sam Aaron in another topic. You could also try syncing your Volca with a DAW and then try to send MIDI or OSC messages from your DAW to Sonic-Pi. These are tricky solutions to setup, but they might be the most efficient.