mod_rewrite interferring with UserDir

Hi,

Im setting up a hosting environment to be used with a new CMS were
building. The server is running DirectAdmin, and Ive setup a few test
users so far to gauge the servers readiness. All seems to be ok - the
sites can be accessed through domains, and also via IP/~user, except
for the test CMS site.

Heres the .htaccess:

RewriteEngine On
RewriteBase /

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)$ dispatch.php [QSA,L]

Accessed via domain, works perfectly. Accessing via IP/~user (which we
will need for site setup before transferring domains) returns the
following:

"Not Found

The requested URL /dispatch.php was not found on this server.
Additionally, a 404 Not Found error was encountered while trying to
use an ErrorDocument to handle the request."

Apache errorlog:
[Fri Feb 29 15:35:55 2008] [error] [client #.#.#.#] script '/var/www/
html/dispatch.php' not found or unable to stat
[Fri Feb 29 15:35:55 2008] [error] [client #.#.#.#] File does not
exist: /var/www/html/404.shtml

Why would it be looking in /var/www/html, and not /home/user/
public_html? Again, this only happens when using the IP/~user access
method, directly to domain name works fine.

Any ideas?