Welcome to the Community - It is not vCenter that sets the MAC Address but the ESXi host generating the MAC address - you can check out page 150 of http://pubs.vmware.com/vsphere-51/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-51-networking-guide.pdf to see how to manually set the MAC address of a VM -
I have also moved this thread to a more appropriate forum