.htaccess conflict with Wordpress

[Dezio]

Hello,
I installed wordpress and vbullettin forum.
wordpress on /.
vbullettin on /forum
If I try to visit http://www.domain.com/forum all work, but if I try to visit http://www.domain.com/forum/topic I've a 404 error.
his is my .htaccess file:
Code:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
  # END WordPress

Can anyone help me?
Regards