Sorting for view model?

I have a view model that combines data from 3 models.

  public class VendorVM
    {
        public IEnumerable<Vendor> Vendors { get; set; }
        public IEnumerable<Promo> Promos { get; set; }
        public IEnumerable<Contact> Contacts { get; set; }
        public IEnumerable<Distributor> Distributors { get; set; }
    }

Here's the controller:

     public async Task<IActionResult> Index(int? id)
            {
ViewData["NameSortParm"] = String.IsNullOrEmpty(sortOrder) ? "name_desc" : "";
            ViewData["DateSortParm"] = sortOrder == "Date" ? "date_desc" : "Date";

           var viewModel = new VendorVM();
                    viewModel.Vendors = await _context.Vendors
                        .Include(i => i.Promos)
                        .Include(i => i.Contacts)
                        .Include(i => i.Distributors)
                        .AsNoTracking()
                        .ToListAsync();
                    if (id != null)
                    {
                        ViewData["VendorID"] = id.Value;
                        Vendor vendorVM = viewModel.Vendors.Where(
                            i => i.VendorID == id.Value).Single();
                        viewModel.Promos = vendorVM.Promos.ToList();
                        viewModel.Contacts = vendorVM.Contacts.ToList();
                        viewModel.Distributors = vendorVM.Distributors.ToList();

 switch(sortOrder)
                {
                    case "name_desc":
                        viewModel.Vendors = viewModel.Vendors.OrderByDescending(i => i.VendorName);
                        break;
                    case "Date":
                        viewModel.Vendors = viewModel.Vendors.OrderBy(i => i.Distributors.OrderBy(v => v.LastUpdateDate));
                        break;
                    case "date_desc":
                        viewModel.Vendors = viewModel.Vendors.OrderByDescending(i => i.Distributors.OrderByDescending(v => v.LastUpdateDate)));
                        break;
                    default:
                        viewModel.Vendors = viewModel.Vendors.OrderBy(i => i.VendorName);
                        break;
                    }
                    return View(viewModel);
    }

How can I add sorting functionality to this view model? I want to sort by VendorName & LastUpdateDate. I'm following this tutorial here: https://docs.microsoft.com/en-us/aspnet/core/data/ef-mvc/sort-filter-page. It doesn't work for my view model as I can only sort Vendors info. I would like to sort promos, contacts & distributors info as well.