NBU 7.6.0.2 does it support V5.5.1?

You may want to subscribe to all the NBU 7.x compatibility lists...

Master Compatibility List: http://www.symantec.com/docs/TECH59978 

To further explain this issue. It is not NetBackup that determine the support per say.

Per the Support for NetBackup 7.x in virtual environments guide,http://www.symantec.com/docs/TECH127089 , at 7.6.0.2 we are using VDDK version 5.5.

Is it then up to what VmWare has determined VDDK 5.5 supports for the rest of the environment.

https://www.vmware.com/support/developer/vddk/vddk-550-releasenotes.html

We had to upgrade to NBU 7.6.0.3 on media servers for compatibility for VM 5.5 Update1A. We are in process of obtaining ET3538035 TECH223790  (internal) for the media servers to see if it will correct high CPU usage on the Vcenter server