Great question. Yes, NetBackup is widely used by many of our customers, and the approach of using the ExternalFreeze/Thaw APIs is the best approach. Also with you environment being on VMware ESXi 6, we also can support using VMDK snapshots as part of the backup process assume you have the feature in NetBackup 8.1 to support VMware guest snapshots. I found the following link from NetBackup 8.1 and their support in a Vmware environment: https://www.veritas.com/content/support/en_US/doc/NB_70_80_VE
You will want to have pre/post scripts added to the NetBackup backup job so that the database is frozen prior to taking the snapshot, and then thawed right after the snapshot. Then NetBackup will take a clean backup of the VMDKs providing an application consistent backup. Here is another link to an article on using ExternalFreeze/Thaw in a VMware environment: https://community.intersystems.com/post/intersystems-data-platforms-and-performance-–-vm-backups-and-caché-freezethaw-scripts
I hope this helps. Please let me know if you have any questions.