You should be looking at Desktop virtualization (VMware Horizon View) and not server virtualization (VMware ESXi & vCenter)...

Esxi On Mac Pro 1 1

Running Virtual desktops on desktops for Dev is a totally different matter from running them in production mode.

Vmware Esxi 6.0 Download

Mac Pro Server. Hypervisor (VMWare Esxi or other) 1 or more Mac OS X Lion Server (or Snow Leopard Server) Virtual Machines. The above architecture is called HW virtualization and we didn't find some thing about it on mac system we only find OS Virtualization that is the following: Mac Pro Server. Virtual Machines (Guest)s. Once completed your USB drive will boot into the ESXi 5 installer. Once booted, install the ESXi 5 Hypervisor to the USB drive (overwriting the installer). This will then leave you with the installed ESXi Hypervisor on the USB. Note that using this method creates a brand new bootable USB key for use in a new installation of vSphere ESXi.

..........taken from http://www.brianmadden.com/opinion/Why-Apple-doesnt-need-to-worry-about-virtualization-of-Mac-OS-X

For those who want the Mac OS X to be able to run in a VM, from a pure technical standpoint, that's possible today. (You just basically hackintosh OS X into a VM.) And actually from a legal standpoint, it appears that it's also ok to run OS X in a VM as long as the VM host is running on Apple-branded hardware. There's even talk that the next version of VMware vSphere will officially support OS X (which we assume is only legal if vSphere was running on Apple hardware).