Arr I see your saying you don't have a current support contract with VMWare or IBM???
Therefore you can't get the ESXi 5.5 u2 installation either IBM version or VMWare version
You really should check your server is on the VMWare HCL before you do this..
vCenter Server can be technically either on a physical piece of hardware or a guest on your ESxi Server.. But in your case I would not have it running as a VM guest.