They are two different technologies. vMotion is moving compute only. Storage vMotion (svMotion) is moving storage. A vMotion that combines both is executed as a svMotion first followed by vMotion second. These can be conducted when "idle" (however you measure that) and "busy" (same). Depending on how well your storage and vMotion networks perform, you may or may not succeed in moving a VM that is under high load.
Thanks! - Didn't realise it was a two part process.
I am currently using a CPU busy bash script, so the VM is at 20% load. So its not what I would call idle, but then not under full load.