NetBackup VxMS logging logging will reveal what is going on when NetBackup calls the VADP and other such VMware APIs:
How to troubleshoot NetBackup for VMWare vSphere backup/restore in NetBackup 7.x
https://www.veritas.com/support/en_US/article.TECH77790
...and maybe search for other "NetBackup VxMS" articles and tech notes.
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bpfis log trouble shooting:
https://www.veritas.com/support/en_US/article.000123692
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There are several sets/groups of "API" functions written and produced by VMware for other third parties (like Veritas) to build in to their applications (like NetBackup). These API sets from VMware are distributed in the form of a VDDK. And it is the VDDK which contains multiple API sets, one of which is VADP. And so, NetBackup is able to read/write the VMFS datastore LUNs directly, because NetBackup calls the approporiate VADP routines from the linked VDDK embedded within the NetBackup binaries.