NetBackup for VMware orphaned snapshots best practice?
What's your Netbackup and vSphere version?
I know of an issue that when you upgraded your Netbackup to 7.6.1.x from older version in a vSphere 4.1 environment, the snapshot removal stop to work properly. The workaround is simply to upgrade your vSphere version, or downgrade your Netbackup version. There was still a confusion and debate of whether 7.6.1.x supports vSphere 4.1 or not, but I guess when the backups keep failing, you will need to go either the upgrade or downgrade way.
Also you can check out more discussions from VMware:
https://communities.vmware.com/thread/430928?db=5
Basically snapshot removal is still a task of VMware, backup software is usually doing the backup and merely being the messenger (to notify create or remove snapshot).