How to validate multiple ViewModels in single view by condition using ModelState.IsValid

I have a problem with validation of one view with multiple ViewModels. My situation is, that I have one Basic form, which is same for lot of pages. By ID parameter, I render new external fields to this Basic form. These external fields are type of ActionResult, using own ViewModel and own Controller. In Main controller on Post action I want to control if ModelState.IsValid, but I have problem - it validate all ViewModels of all external fields, but I want to validate only ViewModel of active external fields (and Basic form too).

It looks like this:

ViewModel of all view models

public class AllFieldsVm
{
    public BasicFormVm BasicFormVm { get; set; }
    public ExternalFieldXyVm ExternalFieldXyVm { get; set; }
    public AnotherExternalFieldVm AnotherExternalFieldVm { get; set; }
}

In controller of external fields I create new instance of AllFieldsVm and in this create new instance of ExternalFieldXyVm (if I need, I prefill these fields). This I render whitout layout like partial view (using @{Html.RenderAction("Action", "Controller", new {@someOptionalData = value});} ), when some condition is true.

In controller of Basic form on Post action I have something like this and I want to use something like this code if (ModelState.IsValid(model.BasicFormVm) && ModelState.IsValid(model.ExternalFieldXyVm)):

[POST("someurl-id{someId}")]
public ActionResult SaveFormData(int someId, AllFieldsVm model)
{
    //Here I want something like 
    //if (ModelState.IsValid(model.BasicFormVm) && ModelState.IsValid(model.ExternalFieldXyVm)) or something like that...

    var se = new SomeEntity();
    se.property1 = model.property1;
    se.property2 = model.property2;

    using (var dbc = _db.Database.BeginTransaction())
    {
        try
        {
            _db.Add(se);
            _db.SaveChanges();

            //My Condition - when save external data
            if (someId == (int) MovementTypes.SomeEnumInt)
            {
                var rd = new ExternalFieldEntity
                    {
                        PropertyA = se.property0,
                        PropertyB = Convert.ToDateTime(model.ExternalFieldXyVm.SomeExternalFieldName)
                    };

                _db.Add(rd);
                _db.SaveChanges();
            }

            dbc.Commit();
        }
        catch (Exception)
        {
            dbc.Rollback();
        }
    }

    return RedirectToAction("Action", "Controller");
}

So, my question is, how can I validate ExternalFieldXyVm separatly based on some conditions?

Is it possible, or I have to create all own validators, without using basic DataAnnotations or FluentValidation? I have no experience with these types of forms, so please be patient...

Thanks to all for help!!