Du verlangst in der Datenbankabfrage das $db_prefix, ohne es irgendwo angegeben zu haben.

PHP-Code:
$stat mysql_fetch_assoc(mysql_query('SELECT k.kontostand,u.nickname FROM 
                           '
.$db_prefix.'_kontodaten AS k
                           LEFT JOIN '
.$db_prefix.'_userdaten AS u ON u.uid = k.uid
                     WHERE k.uid='
.(INT)$_GET['uid'].' LIMIT 1')); 
Ersetze also

PHP-Code:
$db_host        "localhost";                              
$db_user        "user";                                   
$db_pass        "passwort";                                       
$db_base        "datenbank";  

mysql_connect($db_host,$db_user,$db_pass);
mysql_select_db($db_base); 
durch

PHP-Code:
$db_host        "localhost";                              
$db_user        "user";                                   
$db_pass        "passwort";                                       
$db_base        "datenbank";  
$db_prefix        "vms";

mysql_connect($db_host,$db_user,$db_pass);
mysql_select_db($db_base); 
und alles passt dann soweit.

Gruß
Ralf