Create a "must be this page" condition?
 

How can I do something like....

<if page == domain.com/forum/vbnews.php>
display custom stuff here
</if>

?

Is it a vb page? If so there is a variable declared at the beginning of each page that will give you this info.

<if condition="THIS_SCRIPT == 'showthread'">
blah blah
</if>

I think vbnews is a plug-in. Any ideas?

If it's got a page, it's most likely got a THIS_SCRIPT constant -- and if it does not, it could always be added.

How can I find the contant for a THIS_SCRIPT? Is there an equivalent way of echoing it out?

Look in the PHP file, it's usually defined at the top. You could also create a plugin at hook location "global_complete" with the following: