If your hosting account comes with CPanel, you can use the File Manager in there to get to the .htaccess file.
If you cannot get into the .htaccess file. Get you WebHost to set it up so that all requests for mydomain.com permanently redirects to
www.mydomain.com.
Alternatively, run your site without the 'www'. Then go into AdminCP, vBulletin Options-> SiteName / URL / Contact Details and change the Forum URL and Home Page URL so they don't have 'www'. If you have other web software, they'll need looking at too.