While adding an additional vCenter Server to our Multi-Site Single Sign On instance I encountered a problem as I entered the details of the existing SSO.
The error thrown was:
User credentials are incorrect or empty. Provide correct credentials.
After a couple of hours online with VMware support I took a guess at the problem. On the existing Single Sign On Configuration I have added the Active Directory domain DefinIT and in order to enable integrated authentication from the vSphere Client I moved it to the top of the list - this meant that System-Domain is no longer the default authentication domain. The SSO admin account (admin@System-Domain) is a part of that domain and so my guess is that the installer tries to authenticate using firstname.lastname@example.org rather than System-Domain, which of course failed.
Moving System-Domain back to the top of the list allowed me to install correctly, and once finished I could drop it back down to allow integrated authentication again.
VMware vSphere Single Sign On (SSO) can be installed in Multi-site mode to support local sign-on to vCenters that you want to be part of the same single sign on domain - for example, if you want to install Linked-Mode and have the advantage of a single pane of glass view, but can't risk using a single SSO instance across the WAN. In other words, from VMware's blog post:
Multisite deployments are where a local replica is maintained at remote sites of the primary vCenter Single Sign-On instance. vCenter Servers are reconfigured to use the local vCenter Single Sign-On service and reduce authentication requests across the WAN. Multisite deployments do drop the support of single pane of glass views unless Linked Mode is utilized and multisite deployments are actually required to maintain Linked Mode configurations where roles, permissions and licenses are replicated between linked vCenter servers. Linked mode will re-enable single pane of glass views across multisite instances.