Validation best practice for Model and ViewModel

I have separate model and viewmodel classes. Where viewmodel classes only do UI level validation (refer: Validation: Model or ViewModel).

I can verify on post action in a controller that model (vewmodel) is valid.

The ask: How do I validate the model (the main entity with data annotations).

I am not developing the viewmodel using model object. Just duplicating the properties and adding all properties possibly required in that particular view.

//Model Class
public class User
{
    [Required]
    public string Email {get; set;}

    [Required]
    public DateTime Created {get; set;}
}

//ViewModel Class
public class UserViewModel
{
    [Required]
    public string Email {get; set;}

    [Required]
    public string LivesIn {get; set;}
}

//Post action
public ActionResult(UserViewModel uvm)
{
    if( ModelState.IsValid)
        //means user entered data correctly and is validated

    User u = new User() {Email = uvm.Email, Created = DateTime.Now};
    //How do I validate "u"?

    return View();
}

Should do something like this:

var results = new List<ValidationResult>();
var context = new ValidationContext(u, null, null);
var r = Validator.TryValidateObject(u, context, results);

What I am thinking is adding this validation technique in the base class (of business entity), and verify it when I am mapping from viewmodel class to business entity.

Any suggestions?