NameVirtualHost MYIP:80
<VirtualHost MYIP:80>
   ServerName www.MYDOMAIN
   ServerAlias MYDOMAIN
   DocumentRoot /pathto/homedir/
<Directory /pathto/homedir>
   Options -Indexes FollowSymLinks MultiViews
   AllowOverride All
   Order allow,deny
   allow from all
</Directory>
   IndexOptions
   DirectoryIndex index.php index.htm index.html index.shtml index.php3
   RewriteEngine On
   RewriteCond %{HTTP_HOST} ^www.MYDOMAIN$ [NC]
   RewriteRule ^(.*) http://MYDOMAIN/$1 [L,R=301]
   RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} -d
   RewriteRule . - [L]
   RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} -f
   RewriteRule . - [L]
   RewriteCond %{DOCUMENT_ROOT}/users/%{REQUEST_URI} -d
   RewriteRule ^(.+)$ /users/$1/ [L]
   RewriteCond %{DOCUMENT_ROOT}/users/%{REQUEST_URI} -f
   RewriteRule ^(.+)$ /users/$1 [L]
</VirtualHost>


-----this is in another file which becomes included:

<VirtualHost MYIP:80>
        ServerName www.USERNAME.MYDOMAIN
        ServerAlias USERNAME.MYDOMAIN www.USERNAME.MYDOMAIN
        DocumentRoot /pathto/homedir/users/USERNAME
        DirectoryIndex index.php index.php3 index.php4 index.html index.htm
    <Directory /pathto/homedir/users/USERNAME>
            RewriteEngine On
            php_admin_value upload_tmp_dir /tmp
            php_admin_value open_basedir /tmp:/pathto/homedir/users/USERNAME
            Allow from all
    </Directory>
</VirtualHost>