In NetBackup GUI, navigate to Host Properties/Master Servers/
master server/Client Attributes. In the list of clients, look for the client(s) in question & go to the Windows Open File Backup tab. Check to see if the "enable" box is ticked or not & also the "use VSP snapshot provider". If your client is NOT in the list then add it & "switch off" the VSP - this is activated by default, so if client isn't in the list it WILL be using VSP snapshots.
***EDIT***
Sorry, just re-read your post. Your clients aren't in the list so they will be using VSP (if installed). Add them to the list & switch it off.
***EDIT #2***
Or change it to VSS if this is appropriate!
;)