Hi,
its not mandatory for vCenter server to be joined in AD same applies for vCenter server 5.5.
You will just lose some functionality :
VMware KB: Installing vCenter Server 5.5 best practices
If the system that you use for your vCenter Server installation belongs to a workgroup rather than a domain, not all functionality is available to vCenter Server.
If assigned to a workgroup, the vCenter Server system is not able to discover all domains and systems available on the network when using some features.
Your machine must be connected to a domain if you want to add Active Directory identity sources after the installation. To determine whether the system
belongs to a workgroup or a domain, right-click My Computer. Click Properties and click the Computer Name tab. The Computer Name tab displays either
a Workgroup label or a Domain label.
Also have a look at this KB:
VMware KB: Installing vCenter Single Sign-On 5.5 on a Microsoft Windows platform