The problem I had was there were two servers that were brought at different times and as is often the case had slightly different specification based on the deals the vendors were offering at the time. Basically the Intel processors had slightly different capabilities Xeon E5530 and E5620 The problem came up when I tried to vMotion VMs from one box to the other, Virtual Center complained that the older server lacked the PCLMULQDQ and AES-NI CPU features:
|Host CPU is incompatible with the virtual machine's requirments at CPUID level 0x1 register 'ecx'|
So down to the nitty gritty, how do you actually disable these features. Connect the vSphere client, to either your vCenter server or the ESX server itself. Right click the VM you wish to make more mobile and choose "Settings", then on the "Options" tab select CPUID Mask then click on the "Advanced..." tab. Scroll down to the "Level 1" section and for "ecx" the mask you need is (presented here for your copy and paste delight):
---- --0- ---- ---- ---- ---- ---- --0-
This was explained in the KB 1993 article but they refer to the "Level" as "a" which is not mentioned in the ESXi 5.0/vSphere 5 configuration screens.
After I changed this setting all the VMs were happy to v|Motion back and forth between the servers.