The question is, does the licensing refer to the sockets in the host or the vCPUs presented to the VM? In case of the host you may need to either remove one processor or disable it in the BIOS (if possible). In case it's a per VM licensing, you can either present only a single vCPU to the VM or present the vCPUs as a single virtual Multi-Core vCPU (see http://kb.vmware.com/kb/1010184 for details)
André