Hi guys, apologies if this is in the wrong place but unsure where it'd best fit.

I've been asked to re-develop a site for a friend - the site uses Wordpress for it's CMS side, with vBulletin as a community/forum space.

At present, the Wordpress install is using default-style permalinks, which we want changing to another format. Unfortunately, when I try to do this, the site shows a plain "page not found" error.
I've looked into the htaccess file and various help sites, and come to the conclusion (after installing a blank htaccess file) that there's a conflict somewhere in it, but I'm not sure where.

The code in the current file is as follows:
RewriteRule ^((urllist|sitemap_).*\.(xml|txt)(\.gz)?)$ 

vbseo_sitemap/vbseo_getsitemap.php?sitemap=$1 [L]

RewriteCond %{REQUEST_URI} !

RewriteRule ^((archive/)?(.*\.php(/.*)?))$ vbseo.php 


RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !/

RewriteRule ^(.+)$ vbseo.php [L,QSA]

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
Now, I know the first section is something to do with the vBulletin operation, and it's this that I think is conflicting. If I get rid of everything before [L,QSA], the Wordpress install works as it should - but I lose functionality on the forum (posts and links show 404)... If I keep the file above, I lose the Wordpress permalink structure (i.e. only default will work) but the forum works fine.

Any help you guys can offer will be massively appreciated; this is an issue that has been bugging me for weeks (on and off, when I get time to sit down and look at fixing it!)