A vCenter must sync before you can perform operations with it. Unl;ess the vCenter is having issues or is really large it should only take minutes.
If it is "stuck" (and this can happen) you might need to kick the system a little.
http://kickingwaterbottles.blogspot.com/2012/02/reconnecting-vcenter-task-in-vcloud.html
Has the process for shutting down and clearing the schedule tables. This in many cases can get things going again if vCloud just got confused.
Make sure to backup the database to make sure if something goes wrong you have a way to recover.