What are you talking about? If you have vCD 5.1.x on Cent OS 5.x, then just use the installer you have. there is only one installer for all scenarios (it does fresh installs and upgrades). the only other way to get it is an appliance (but you wouldn't use that in this case).
You don't even uninstall vCD 5.1.x ... just run the new installer bin on the CentOS system with vCD 5.1.x installed and it'll detect that it's an upgrade situation and go forward. Since the DB looks to have already been upgraded, you can skip that part and start the cell services once the upgrade is completed.
If you really want to upgrade CentOS to 6.x generation, then you would upgrade vCD first then CentOS second (ideally with the vCloud services stopped).