If you are getting the 5.1 product set, the vCloud Suite license key will unlock vCloud Director, ESXi, and vCloud Network/Security (vShield). So, one key will work for all.
In the older versions of vCloud Director, like 1.0 and 1.5, there was a 'vShield Free' version that was included with vCloud Director ... this was to support the required basic parts of vShield to make Edge Firewalls/NATs