The only other thing I can think is related to the version of VM itself.
It seems to me that the VM compatibility version has to be 7 or above to use vMotion in 5.x hosts, but I can't swear to that.
If you look at the VM's summary tab in the web client, what version is listed?
That said, the error message usually tells you if the machine version is at issue, whereas your error didn't mention it.
If that's not the issue, then it may be time to put that tech support you are paying for to work.