VMware Vcenter 5.1 and before the default behaviour was to allow all domain admins to login and administrate your ESXi hosts. Once the 5.5 vCenter installed if you try and connect using a domain account you will get the error :The vSphere Client could not connect to “FQDN / IP”. You do not have permission to login to the server: FQDN / IP
To get domain credentials to work you need to login to the VMware vSphere Web Client and setup the relevant permissions.
So how to login to vCenter using web client if domain users are not allowed?
Well that will be the one you configured when you installed vCenter and known as the vCenter Single Sign on.
From my case the login would be firstname.lastname@example.org and the password would be what I entered. Insert these details into the VMware vSphere Web Client and click Login.
Once logged in you will see the vCenter Home.
Click on vCenter on the left hand side , Then click on vCenter Servers
Click on your vCenter server on the left. Then click manage on the right.
Click on the permissions button
Then add your relevant groups / users who you want to administrate your ESXi hosts.
Set the privileges to the user
Now you are allowed to login using that particular account to access vCenter Server