Mvc - Form with viewmodel

im working in mvc and have a form in my view which receive from my viewmodel a selectlist to insert the values into my dropdownlist.

but on submit the form nothing work and i can´t figured it out what its going on.

on submit the form i get null into my database.

someone can help me out with this pls?

here is my code: VIEW:

model MyProject.Models.ViewModel
@using(Html.BeginForm()) {
    @Html.DropDownListFor(o => o.cp.Companylist,Model.cp.Companylist)
     @Html.EditorFor(o => o.md.title)
     @Html.EditorFor(o => o.md.edition)
    @Html.EditorFor(o => o.md.number)

  <input type="submit" name="submit" value="Add New">
}

Controller:

  public ActionResult Create()
        {   ViewModel vm = new ViewModel ();
            return View(vm);
        }

        //
        // POST: Create

        [HttpPost]
        public ActionResult Create(Mydata model)
        {
            if (ModelState.IsValid)
            {
                db.Res.Add(model);
                db.SaveChanges();
                return RedirectToAction("Index","AdminPanel");
            }

            return View(model);
        }

Model:

 public class ViewModel
    {
        public Mydata md { get; set; }
        public Company cp { get; set; }
    }

Mydata - Model:

 public class Mydata
    {
        public int ID { get; set; }
        public string company_name{ get; set; }
        public string title{ get; set; }
        public string edition{ get; set; }
        public string number{ get; set; }
    }
public class DefaultConnection : DbContext
    {
        public DbSet<Mydata> Res{ get; set; }
    }

Companies - Model:

 public class Company
    {
        public int id { get; set; }
        public string name{ get; set; }
        public SelectList Companylist{ get; set; }  

        public Company()
        {
            Companylist= GetCompanies();
        }

        public SelectList GetCompanies()
        {
            var list = new List<SelectListItem>();
            string connection = ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString;

            using (var con = new SqlConnection(connection))
            {
                con.Open();
                using (var command = new SqlCommand("SELECT * FROM MyCompanies", con))
                {
                    SqlDataReader reader = command.ExecuteReader();
                    while (reader.Read())
                    {
                        //string id = reader[0] as string;
                        string name_company= reader[1] as string;
                        list.Add(new SelectListItem() { Text = name_company, Value = name_company});
                    }
                }
                con.Close();
            }
            return new SelectList(list, "Value", "Text");
        }
}