For the most part, discussing how to secure vCenter Server entails discussing how to secure Windows Server. As a Windows-based application, vCenter Server relies upon the underlying security features of Windows Server. As a result, a couple of the security recommendations I'll make are common to any Windows Server installation:
Stay current on all Windows Server and vCenter Server patches and updates. This helps protect you against potential security exploits.
Harden the Windows Server installation using published best practices and guidelines from Microsoft. This is true not only for the vCenter Server computer but also for the computer running the separate database server in support of vCenter Server.
A few security ...