So I had some time to try the vst version of the VC with ableton live this evening. It is certainly a lot more stable than the rtas version, the arp's sync & the latency is negligible. There is still some crackling if I play around with a pot a lot but it is at least useable. I think I will load up my ROM banks with the new patches for 5 but just use it without VC because if it has taken a whole year to produce this then who know's when if ever there will be a stable version.
It seems that if you want to use VC you have to buy a PC & use Cubase
anything else is a 50/50 chance of working properly.