Trying to track down issue with custom ErrorPage

[محروم.كوم]

I keep trying to track down some .htaccess issues with forum. When I enter a bogus URL on my home page, I get my custom error page displayed. But inside VB I do not...:

www.yoursite.com/fileDoesNotExist.html
404 error page is displayed correctly


www.yoursite.com/forum/fileDoesNotExist.html
only the work "Page Not Found" is printed. No other HTML or anything.

I am using vbSEO with this .htaccess:

Code:
ErrorDocument 404 http://www.blincmagazine.com/404.php

# Comment the following line (add '#' at the beginning)
# to disable mod_rewrite functions.
# Please note: you still need to disable the hack in
# the vBSEO control panel to stop url rewrites.
RewriteEngine On

# Some servers require the Rewritebase directive to be
# enabled (remove '#' at the beginning to activate)
# Please note: when enabled, you must include the path
# to your root vB folder (i.e. RewriteBase /forums/)
RewriteBase /forum/

##############################
#RewriteCond %{HTTP_HOST} !^www\.yourdomain\.com
#RewriteRule (.*) http://www.yourdomain.com/forum/$1 [L,R=301]

RewriteRule ^((urllist|sitemap_).*\.(xml|txt)(\.gz)?)$ vbseo_sitemap/vbseo_getsitemap.php?sitemap=$1 [L]

RewriteCond %{REQUEST_URI} !(admincp/|modcp/|cron|vbseo_sitemap|wiki|showwiki)
RewriteRule ^((archive/)?(.*\.php(/.*)?))$ vbseo.php [L,QSA]


RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !/(admincp|modcp|clientscript|cpstyles|images|wiki|s howwiki)/
RewriteRule ^(.+)$ vbseo.php [L,QSA] __DEFINE_LIKE_SHARE__