Have a look at NBU 8 - 8.x OS SCL : https://www.veritas.com/content/support/en_US/article.100040093
In the VMware Compatibility section, the NBU -> VDDK -> vSphere versions are listed.
The SCL also says to 'Refer VMware VDDK release notes URL below ... '
According to the documentation NBU 8 - 8.x OS SCL and Refer VMware VDDK release notes, I have seen that it does not comment on the version of vCenter Server Appliance 6.5 (VCSA), but on the esxi host version.
I understand that if the VCSA 6.5u2 virtual machine is inside an esxi 5.5u3 netbackup 8 host it will use vddk 6.0.2.
If I have understood correctly the netbackup is based on the esxi physical host and not so much of the Center Server Appliance.