> I'm finding the open file thing a bit hit and miss to be honest. 

Demystification summary:  VSP is "default: on" only if you install it
(why would you install it and want it to default to off?) and the
installation of VSP's predecessor included a boot to make it active.

You first control VSP by including it (the default) during installation.
OTM (4.5 and earlier) was the same way.  Haven't installed a Win client
in ages, but it is one of those little installation GUI blocks that have
a "do not install" option.  From the Installation Guide for Windows:  

  during the installation process you have the option of
  installing VERITAS Volume Snapshot Provider (VSP)

> Since I first started this thread, I've found out that VSP
> is setup for everything by default unless otherwise specified
> (according to admin guide); 

>From that admin guide or completeness:  

  By default, [...] the server uses the following Windows
  Open File Backup defaults for all Windows NetBackup
  clients (5.0 or later):

  o  Windows Open File Backup is enabled on the client.
  o  The snapshot provider for the client is VSP. [...]

> Perhaps the power outage made VSP work?

If this were OTM (older NetBackup equivalent of VSP), I'd say it was the
reboot.  Including OTM in the client installation generated a deferrable
reboot after installation so it could load as a device driver.  The
NetBackup 6.0 Installation Guide says

  Note  You may need to reboot the system for the changes to
  take effect.

Until demonstrated otherwise, I'd expect VSP to behave like OTM WRT
needing a boot to set up properly.  Is it possible that the boxes that
had WOFB issues weren't booted after client installation?