Hi...
IMHO, I think it's because that the required CPU mask for that family of CPUs (Westmere) not supported in HW level 7 or 8.
It's supported only by 9.
U can try one of two things beside what u did:
1-) put EVC back to Westmere Family and try to change a VM with HW level 7 or 8 mask manually according to this KB:
OR...
2-) disable this feature (AES Feature) on all of your physical hosts CPUs then try again to put the EVC level to Westmere and try to vMotion again a VM with HW level 7 or 8.
Waiting for your results..