From http://www.vmware.com/download/eula/virtualcenter.html
VMware grants you a nonexclusive, nontransferable license, without rights to sublicense, to :smileyinfo: use the Software License Keys to activate Servers for management by the VirtualCenter Management Server Software and/or management by the Additional Module; (ii) install or have installed a single instance of the VirtualCenter Management Server Software on a single Server, unless permitted to have multiple instances on a single Server or to have multiple instances on multiple Servers by the payment of applicable license fees[/b] (whether such fees are based on a per Processor, a per Virtual Machine, a per user or any other VMware approved licensing model);...[/i]
You need to license each VC instance.[/b]
Disclaimer: I am not a lawyer, nor do I play one on TV.