Delegate Task web services invoking using Jquery

Hi Team, 

Any one please suggest calling delegate task web services using jquery, getting error  "internal server error". Below is my code and find attached form.

function updateassigntowebservicecall()
{
vdelegated = false;
var vvalidation = true;

$('#lblerrmsg').empty();
var vassignto = $('#ddlDelegate').val();
var vcomments = $('#txtComments').text()

if (vassignto == "**SelectValue**")
{
$('#lblerrmsg').text('Please select Delegate.');
vvalidation = false;
}
if (vcomments == "")
{
$('#lblerrmsg').text('Please select Comments.');
vvalidation = false;
}

if (vvalidation == true) {

if (parseInt(vworkfowtaskid) > 0) {

var new_string = "";
new_string += '<?xml version="1.0" encoding="utf-8"?>';
new_string += '<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope" xmlns:nin="http://nintex.com">';

new_string += '<soap:Header/>';
new_string += '<soap:Body>';
new_string += '<nin:DelegateTask>';
new_string += '<nin:spTaskId>'+ vworkfowtaskid +'</nin:spTaskId>';
new_string += '<nin:taskListName>Workflow Tasks</nin:taskListName>';
new_string += '<nin:targetUsername>' + vassignto + '</nin:targetUsername>';
new_string += '<nin:comments>' + vcomments + '</nin:comments>';
new_string += '<nin:sendNotification>true</nin:sendNotification>';
new_string += '</nin:DelegateTask>';
new_string += '</soap:Body>';
new_string += '</soap:Envelope>';

alert(new_string);
callwebservicesfordelegation(new_string);

}

}

}

function callwebservicesfordelegation(vwebsericebatch) {

var wsUrl = sitefullurl + "_vti_bin/NintexWorkflow/Workflow.asmx";
$.ajax({
type: "POST",
url: wsUrl,
dataType: "xml",
data: vwebsericebatch,
success: function (data, status, req) {
if (status == "success") {
alert("hi");
}
},
contentType: "text/xml; charset=utf-8",
error: function (requestObject, error, errorThrown) {
alert(requestObject.status + " " + error + " " + errorThrown);
}

});
}