The preferred way to do this would be to make the search extensible, meaning that it runs through the forum tables by default, but that admins could turn on extension to the search which would cause it to search other tables (like classifieds, or gallery, garage, whatever) for the same text. The issue is that either vbulletin would have to maintain a list of tables that needed searching for various mods, or to extend the search the admin would enter these parameters into the search extension config box in admincp and then it would extend per however many boxes were filled in. Vbulletin could allow say up to 10 extensions or maybe even make that relational and have as many as the admin needs.
yes my form is a bit of trickery by altering the search template, actually it is the blog search template because that gave me the idea and was easiest to modify.
It works, it just requires the user to fill in each box but at least they can do it from a centralized location, it just isn't a single extensible search. I could probably write that mod if I had the time.
One other issue, of course, is if your search finds something in another table do you just bring back the field it was found in? In which case that might not be too helpful, but instead if you could configure the fields associated with that record and then bring back either the entire record or just whatever the admin configured for that record that would be really slick.
In all honestly the vbulletin guys could crank out this mod in an afternoon, not really sure why they haven't other than lack of interest...