Validation is not working in MVC 4 drop down list

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validation

MVC4

This is my "view model".

//Vendor  info
      public string VendorName { get; set; }
      [Display(Name = "Vendor")]
      [Required(ErrorMessage = "Please select a Vendor")]
      public Guid VendorId { get; set; }

      //Vendor list
      public IEnumerable<IpNetworkViewModel> GetVendor(Guid Cid)
      {
          var SQry = from l in db.Vendors
                     where l.CustomerId == Cid
                     orderby l.VendorName
                     select new { l.VendorId, l.VendorName };

          List<IpNetworkViewModel> SList = new List<IpNetworkViewModel>();
          SList.Add(new IpNetworkViewModel { VendorId = Guid.Empty, VendorName = "-Select Vendor-" });

          foreach (var p in SQry)
          {
              SList.Add(new IpNetworkViewModel { VendorId = p.VendorId, VendorName = p.VendorName });
          }
          return SList;

      }

This is my "View"

<div class="all-div">
           @Html.LabelFor(model => model.VendorId)
        </div>
         <div class="all-div">
          <div class="drop_down">
           @Html.DropDownListFor(model => model.VendorId, new SelectList(new ConsultantsHub.Areas.ChAdmin.ViewModel.IpNetworkViewModel().GetVendor(Model.CustomerId), "VendorId", "VendorName"))
           @Html.ValidationMessageFor(model => model.VendorId)
        </div>
        </div>

Any one, please help me how to do required field validation for above drop down list.