[Chadi]

Thanks Ed. Still confused though.

The plugin now is this:

<?xml version="1.0" encoding="ISO-8859-1"?>

<product productid="asp_thread_listing" active="1">
    <title>Thread Listing by Letter</title>
    <description>Listing of threads by the first letter of the title</description>
    <version>.1</version>
    <url />
    <versioncheckurl />
    <dependencies>
    </dependencies>
    <codes>
    </codes>
    <templates>
    </templates>
    <plugins>
        <plugin active="1" executionorder="5">
            <title>Add Where Letter</title>
            <hookname>forumdisplay_query</hookname>
            <phpcode><![CDATA[if(isset($_GET['letter'])){
$vbulletin->input->clean_GPC('r', 'letter', TYPE_STR);

$letter = $db->escape_string($vbulletin->GPC['letter']);

$hook_query_where .= " AND LEFT(thread.title, 1) = '$letter'";

}]]></phpcode>
        </plugin>
    </plugins>
    <phrases>
    </phrases>
    <options>
    </options>
    <helptopics>
    </helptopics>
    <cronentries>
    </cronentries>
    <faqentries>
    </faqentries>
</product>

With your changes, is it supposed to look like this?

<?xml version="1.0" encoding="ISO-8859-1"?>

<product productid="asp_thread_listing" active="1">
    <title>Thread Listing by Letter</title>
    <description>Listing of threads by the first letter of the title</description>
    <version>.1</version>
    <url />
    <versioncheckurl />
    <dependencies>
    </dependencies>
    <codes>
    </codes>
    <templates>
    </templates>
    <plugins>
        <plugin active="1" executionorder="5">
            <title>Add Where Letter</title>
            <hookname>forumdisplay_query</hookname>
            <phpcode><![CDATA[if(isset($_GET['letter'])) {
    $vbulletin->input->clean_GPC('r', 'letter', 'TYPE_STR);
    $letter = $db->escape_string($vbulletin->GPC['letter']);

    if($letter == '#') {
        $hook = " AND LEFT(thread.title, 1) BETWEEN 0 AND 9";
    }
    else {
        $hook = " AND LEFT(thread.title, 1) = '$letter'";
    }

    $hook_query_where .= $hook;
}]]></phpcode>
        </plugin>
    </plugins>
    <phrases>
    </phrases>
    <options>
    </options>
    <helptopics>
    </helptopics>
    <cronentries>
    </cronentries>
    <faqentries>
    </faqentries>
</product>

I'll look into the vbseo thing again.

--------------- Added [DATE]1240808021[/DATE] at [TIME]1240808021[/TIME] ---------------

Just wanted to give you a heads up. Tested your code on a fresh install, non-vbseo. It always displayed all threads regardless of the letter clicked.