[Dismounted]

The vBulletin templating system is actually quite flexible easy to use (once you get the hang of it). Add this to your template:

<script type="text/javascript">
    //<![CDATA[
    var myVar1 = <if condition="isset($myPhpVar1)">$myPhpVar1<else />$myPhpVar1Default</if>;
    var myVar2 = <if condition="isset($myPhpVar2)">$myPhpVar2<else />$myPhpVar2Default</if>;
    var lenght = 25;
    
    function func(){
        if (length > $lengthPHP)
        {
            document.getElementById('myID').value = document.getElementById('myID').value.substring(0, $lengthPHP);
        }
    }
    //]]>
</script>

BTW, you spelt "length" wrong. I've corrected this in your code (you will need to adjust your other code, potentially).