Vendors
-------------------
VendorID (PK)
Name

Regions
-------------------
RegionID (PK)
Name

VendorRegions
-------------------
VendorID (PK)
RegionID (PK)

public class VendorsController : Controller
{
    readonly IVendorsRepository _vendorsRepository;
    readonly IRegionsRepository _regionsRepository;

    public VendorsController()
    {
        _vendorsRepository = new SqlVendorsRepository();
        _regionsRepository = new SqlRegionsRepository();
    }

    [HttpPost]
    public ActionResult Create(VendorViewModel viewModel, string[] Regions)
    {
        var vendor = new Vendor();
        TryUpdateModel(vendor);

        if (ModelState.IsValid)
        {
            vendor.Regions.Clear();
            foreach (var regionId in Regions)
            {
                vendor.Regions.Add(_regionsRepository.GetRegion(Int32.Parse(regionId)));
            }

            _vendorsRepository.SaveVendor(vendor);
            return RedirectToAction("Index");
        }

        return View(viewModel);     // validation error, so redisplay same view
    }
}

public class SqlVendorsRepository : IVendorsRepository
{
    private readonly MyDBEntities _entities;

    public SqlVendorsRepository()
    {
        _entities = new MyDBEntities();
    }


    public void SaveVendor(Vendor vendor)
    {
        // If it's a new vendor, just attach it to the DataContext
        if (vendor.VendorID == 0)
            _entities.Vendors.Context.AddObject("Vendors", vendor); // ERROR HERE
        else if (vendor.EntityState == EntityState.Detached)
        {
            // We're updating an existing vendor, but it's not attached to this data context, so attach it and detect the changes
            _entities.Vendors.Context.Attach(vendor);
            _entities.Vendors.Context.Refresh(System.Data.Objects.RefreshMode.ClientWins, vendor);
        }
        _entities.Vendors.Context.SaveChanges();
    }
}

public class SqlRegionsRepository : IRegionsRepository
{
    private readonly MyDBEntities _entities;

    public SqlRegionsRepository()
    {
        _entities = new MyDBEntities();
    }


    public IQueryable<Region> Regions
    {
        get { return _entities.Regions.AsQueryable(); }
    }


    public Region GetRegion(int id)
    {
        return Regions.FirstOrDefault(st => st.RegionID == id);
    }
}

MyDBEntities entities = new MyDBEntities();
_vendorsRepository = new SqlVendorsRepository(entities);
_regionsRepository = new SqlRegionsRepository(entities);

MyDBEntities entities = new MyDBEntities();
_vendorsRepository = new SqlVendorsRepository(entities);
_regionsRepository = new SqlRegionsRepository(entities);