_LoginForm.cshtml

@using NetMvcExample.Models

@model LoginViewModel

<style type="text/css">
	.margin-top-2 {
		margin-top: 2vh;
	}
</style>

<div id="form-placeholder">
	@using (Ajax.BeginForm("Login", "Account", new { }, new AjaxOptions
	{
		HttpMethod = "POST",
		OnBegin = "loginStarted",
		OnSuccess = "loginSuccess",
		OnFailure = "loginFailed",
		InsertionMode = InsertionMode.Replace,
		UpdateTargetId = "login_form_container"
	}, new { @id = "loginForm" }))
	{
		@Html.ValidationSummary(true, null, new { @class = "alert alert-danger" })

		<div class="row">
			<div class="col-md-12">
				<div class="row margin-top-2">
					<div class="col-md-2">
						<span>User Name:</span>
					</div>
					<div class="col-md-8">
						@Html.TextBoxFor(m => m.UserName, new { @class = "k-textbox" })
						@Html.ValidationMessageFor(m => m.UserName)
					</div>
				</div>
				<div class="row margin-top-2">
					<div class="col-md-2">
						<span>Password:</span>
					</div>
					<div class="col-md-8">
						@Html.PasswordFor(m => m.Password, new { @class = "k-textbox" })
						@Html.ValidationMessageFor(m => m.Password)
					</div>
				</div>
				<div class="row margin-top-2">
					<div class="col-md-2">
						<span>Message:</span>
					</div>
					<div class="col-md-8">
						@Html.TextAreaFor(m => m.Message, new { @class = "k-textbox", rows = "5", cols = "30" })
						@Html.ValidationMessageFor(m => m.Message)
					</div>
				</div>
				<div class="row margin-top-2">
					<div class="col-md-2">
						<button type="submit" class="btn btn-primary pull-left">Отправить</button>
					</div>
				</div>
			</div>
		</div>
	}

	<script type="text/javascript">
	
		$(document).ready(function () {
			var loginForm = $("#loginForm");
			loginForm.on("submit", function (e) {
				e.stopImmediatePropagation();
				e.preventDefault();
				
				var validator = loginForm.validate();
				if (!validator.form()) {
					validator.destroy();
					return;
				} else {
					validator.destroy();
				}

				$.ajax({
					url: e.target.getAttribute("action"),
					method: e.target.getAttribute("method"),
					contentType: "application/x-www-form-urlencoded",
					data: loginForm.serialize(),
					type: "json",
					beforeSend: function (xhr) {
						xhr.setRequestHeader("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8");
					},
					success: function (data, status, xhr) {
						console.log(xhr.responseText);

						$("#login_form_container").replaceWith(xhr.responseText);
					},
					error: function (xhr, error) {
						console.log(xhr.responseText);

						if (xhr) {
							if (xhr.status === 302) {
								window.location.href = xhr.responseText;
							}
						}
					}
				});
			});
		});
		
	</script>
</div>

AccountController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Threading.Tasks;
using System.Web.Mvc;

using NetMvcExample.Models;

namespace NetMvcExample.Controllers
{
	public class AccountController : Controller
	{
		[HttpGet]
		public ActionResult Login()
		{
			return View(new LoginViewModel());
		}

		[HttpPost]
		public async Task<ActionResult> Login(LoginViewModel viewModel)
		{
			if (!ModelState.IsValid)
			{
				return PartialView(viewModel);
			}

			await Task.Delay(2000).ConfigureAwait(false); // emulate long work
			
			return new CustomStatusCodeResult(HttpStatusCode.Redirect, Url.Action("Index", "Home"));
		}

		private ActionResult BadRequest(ModelStateDictionary modelState)
		{
			var errors = modelState.Values
				.Where(x => x.Errors.Count > 0)
				.SelectMany(x => x.Errors)
				.Select(x => x.ErrorMessage);

			var errorsText = string.Join("\r\n", errors);

			return new CustomStatusCodeResult(HttpStatusCode.BadRequest, errorsText);
		}
	}
}

CustomStatusCodeResult.cs

using System;
using System.Net;

namespace System.Web.Mvc
{
	public class CustomStatusCodeResult : HttpStatusCodeResult
	{
		private readonly string _content;

		public CustomStatusCodeResult(HttpStatusCode statusCode, string content) : base(statusCode)
		{
			_content = content;
		}

		public override void ExecuteResult(ControllerContext context)
		{
			base.ExecuteResult(context);

			context.HttpContext.Response.Write(_content);
		}
	}
}

Login.cshtml

@using NetMvcExample.Models

@model LoginViewModel

<div id="login_form_container">
	@Html.Partial("_LoginForm", Model)
</div>

LoginViewModel.cs

using System;
using System.ComponentModel.DataAnnotations;

namespace NetMvcExample.Models
{
	public class LoginViewModel
	{
		[Required]
		[DataType(DataType.Text)]
		public string UserName { get; set; }

		[Required]
		[DataType(DataType.Password)]
		public string Password { get; set; }

		[Required]
		[DataType(DataType.Text)]
		public string Message { get; set; }
	}
}