[Angel-Wings]

Originally Posted by James Birkett

Can I allow that handler to only run on sub-domains to stop PHP parsing on them sub-domains? I still host a vBulletin forum on the same server - so, of course, I don't want to stop PHP altogether.

Depends on how your PHP is set. If you run it via CGI or FastCGI - usually you've an Suexec Wrapper - something like:

Global HTTP Conf:

FastCgiWrapper /usr/local/sbin/suexec

And then place in your Vhosts:

<VirtualHost ....>
FastCgiServer /usr/local/example/php_vhost_1 -user domain_1 -group domain_2
...
</VirtualHost>

Using this you've PHP running under different users for each Vhost. If that line is missing, no PHP will be executed.
Just look how you can set FastCGI with an Suexec Wrapper.

If you use the Module:

<VirtualHost ...>
php_admin_flag engine off
php_admin_value open_basedir /nonexistent
</VirtualHost>

Will do it. The second one isn't required, just to be safe setting open_basedir to a non existing directory never hurts.
Or you do it vise versa and disable PHP globally by php.ini then enabling it only for the Vhosts you will need it.