As far as I understand it, you don't need to specify the USERS for VSIFAX. VSIFAX has user maintenance as part of it's Client/Server model where you can fax from PC-based apps. For your standard UNIX-based jobs, there should be no need to manage individual users.
But, since you have a 50-user license, are your faxes originating at the Client workstation?
As PHV said, check the printer I/F script. Your earlier post listed a command for initiating the fax job. Was that command part of the I/F script? As a non-root user, just issue an 'lp' command to that pseudo printer and see if you get an error. Check permissions on the I/F script and related spooler files.
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