WordPress MU

I tried to change my .htaccess so that my forums subdomain would work by adding the following code:

RewriteCond %{HTTP_HOST} forums.domain.com
RewriteCond %{REQUEST_URI} !forums/
RewriteRule ^(.*)$ forums/$1 [L]

However, when I added this, I got a 500 Error. So I put in a ticket with my VPS host to see what the issue was (since I'm not a coding guru) and they said that they removed RewriteBase BASE from the .htaccess, and showed me the error log that said "RewriteBase: argument is not a valid URL."

Now my .htaccess looks like this (without the RewriteBase BASE)...is this going to cause MU problems?

RewriteEngine On

#uploaded files
RewriteRule ^(.*/)?files/$ index.php [L]
RewriteCond %{REQUEST_URI} !.*wp-content/plugins.*
RewriteRule ^(.*/)?files/(.*) wp-content/blogs.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteCond %{REQUEST_URI} ^.*/wp-admin$
RewriteRule ^(.+)$ $1/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule . - [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(wp-.*) $2 [L]
RewriteRule  ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

<IfModule mod_security.c>
<Files async-upload.php>
SecFilterEngine Off
SecFilterScanPOST Off
</Files>
</IfModule>

Check that...it makes a huge difference removing Rewrite Base...I couldn't access my wp-admin...

I added it back, now all is good again...but now I'm hesitant to add the code again to allow my forums subdomain to work correctly...

Is the code I have correct? If so, where exactly should I add it?

In .htaccess there should be

RewriteBase /

AND IN NO CASE (!!!)

RewriteBase BASE