I recently purchased and installed vBSEO, a "friendly URL" and search engine optimization add-on for the vBulletin discussion/community software. The software does a lot of things, including making use of various URL rewrites. It's working great, except -- my site's 404 page is not displaying for the folder vBulletin resides in/runs from. Instead of my custom 404 page, which comes up elsewhere on my site, I get a non-HTML, unstyled, "Page not found" (plain text) message displayed in my browser for files not found in the folder.

Example:
http://www.example.com/missingfile correctly displays my custom 404, but http://www.example.com/vbulletin/missingfile just displays "File not found".

I'm on a shared hosting plan and have my custom server messages (404, etc.) configured in my cpanel. Nothing's changed there recently... that I know of (recently migrated servers).

I'm not "blaming" vBSEO, I just mention it because it seems like the logical place to start...

Any thoughts? Do I need to share a (cleaned up) version of my .htaccess file for the folder that's having difficulties?