Failed to verify the service account. Error code:  - when installing vCenter
While installing vCenter 5.1 recently I experienced the following error at the point where you confirm the AD account to use for the vCenter service:
Failed to verify the service account. Error code: 
At first I figured I was mis-entering the credentials, but it turns out to be an issue if the user AD account for the vCenter service is the same name as the vCenter computer account.
Renamed the AD account and started the install again.