Not getting any data while all parameters passed through ajax jquery call are null

city/area/vendortype

<script type="text/javascript">
        $(document).ready(function () {
            var currentPageNumber = 1;
            loadData(currentPageNumber);

            $(window).scroll(function () {
                if ($(window).scrollTop() == $(document).height() - $(window).height()) {

                    currentPageNumber += 1;
                    loadData(currentPageNumber);
                }

            });

            function loadData(currentPage) {
                var parcity=$("#lblCityParameter").val();
                var pararea = $("#lblAreaParameter").val();
                var parvendortype = $("#lblVendortype").val();
                $.ajax({
                    type: 'post',
                    url: 'WebService/VendorService.asmx/GetVendors',
                    data: { pageNumber: currentPage, pageSize: 6, city:parcity, area:pararea, vendortype:parvendortype },
                   dataType: 'json',
                   //contentType: "application/json; charset=utf-8",
                     success: function (data) {
                      var vendorRepeater = $('#repdata');
                      $(data).each(function (index, ven) {
                         
                            vendorRepeater.append('<div class="col-md-4 wow fadeInLeft animated" data-wow-delay="0.4s" style="text-align:center"><div id="Div1" class="living_boxauto" runat="server"><a href="/KB/answers/VendorDetails.aspx"?VendorId=' + ven.pID + '"><div class="media"><img runat="server" class="img-responsive" src="images/'
                                + ven.pcoverimage +
                                '" style="width:300px;height:200px;" /><div class="media__body"><h2>Image Title</h2><p>Description</p></div></div></a><div id="Div2" class="living_desc" runat="server"><h3><a href="/KB/answers/VendorDetails.aspx"?VendorId='
                                + ven.pID +
                                '"><asp:Label ID="lblVendorName" runat="server" Text="'
                                + ven.pvendorname +
                                '" CssClass="lblVendorName"></asp:Label></a></h3><p></p></div></div></div>');
                        });
                    }
                });

            }

        });

    </script>

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line. 
[System.Web.Script.Services.ScriptService]
public class VendorService : System.Web.Services.WebService {

    public VendorService () {

        //Uncomment the following line if using designed components 
        //InitializeComponent(); 
    }


    //   public void GetVendors(int pageNumber, int pageSize)
    [WebMethod]
 //  [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
  public void GetVendors(int pageNumber, int pageSize, string city, string area, string vendortype)
  {
        List<VendorData> listVendors = new List<VendorData>();

        string cs = ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString;
        using (SqlConnection con = new SqlConnection(cs))
        {
            SqlCommand cmd = new SqlCommand("spGetVendorbyFilter", con);
            cmd.CommandType = System.Data.CommandType.StoredProcedure;
            cmd.Parameters.AddWithValue("@PageNumber", pageNumber);
            cmd.Parameters.AddWithValue("@PageSize", pageSize);
            
            cmd.Parameters.AddWithValue("@city", city);
            cmd.Parameters.AddWithValue("@area", area);
            cmd.Parameters.AddWithValue("@vendortype", vendortype);
            

            con.Open();
            SqlDataReader rdr = cmd.ExecuteReader();
            while (rdr.Read())
            {
                VendorData vendor = new VendorData();
                vendor.pID = Convert.ToInt32(rdr["VendorID"]);
                vendor.pvendorname = Convert.ToString(rdr["VendorName"]);
                vendor.pcoverimage = Convert.ToString(rdr["CoverImage"]);
                vendor.pcategory = Convert.ToString(rdr["Category"]);
                listVendors.Add(vendor);

            }
        }

        JavaScriptSerializer js = new JavaScriptSerializer();
        Context.Response.Write(js.Serialize(listVendors));
    }
    
}

ALTER PROCEDURE [dbo].spGetVendorbyFilter 
                      @PageNumber INT,
                      @PageSize   INT,
                      @city       VARCHAR(200),
                      @area       VARCHAR(200),
                      @vendortype VARCHAR(200)
AS
  BEGIN
      DECLARE @StartRow INT
      DECLARE @EndRow INT

      SET @StartRow = ( ( @PageNumber - 1 ) * @PageSize ) + 1;
      SET @EndRow= @PageNumber * @PageSize;



WITH Result
           AS (
               SELECT *,
                      Row_number()
                        OVER (
                          ORDER BY VendorID ASC) RowNumber
               FROM   tblVendor WHERE (@city IS NULL OR (City = @city))
                 AND  (@area IS NULL OR (Area = @area))
                 and  (@vendortype IS NULL OR (Category = @vendortype))
               
              ) 
                  
     Select  * from Result
     WHERE RowNumber BETWEEN @StartRow And @EndRow

END