ViewModel does not update Model correctly on submit

Question

I am trying to use a strongly-typed ViewModel and find that when I replace the Model class with a ViewModel, changes are not submitted correctly from the Edit template. Whereas just using the straight Model class in the View, edits happen successfully. My model class is MaterialDefinition and the ViewModel class is MaterialDefinitionViewModel as shown below. I've updated the edit template correctly to reference the ViewModel but as I say editing just does not work. I'm using VS2013 and MVC4. Any ideas, someone?

First the ViewModel class...

public class MaterialDefinitionViewModel
{
    // Properties
    public MaterialDefinition Definition { get; private set; }

    // Constructor
    public MaterialDefinitionViewModel(MaterialDefinition def)
    {
        Definition = def;
    }
}

and now some code from the View...

<div class="editor-field">
    @Html.EditorFor(model => model.Definition.mdDescription)
    @Html.ValidationMessageFor(model => model.Definition.mdDescription)
</div>

<p>
    <input type="submit" value="Save" />
</p>

Answer 1

Finally discovered the answer to this after much searching...

    [HttpPost]
    public ActionResult Edit(string id, FormCollection collection)
    {
        MaterialDefinition def = repository.GetMaterialDefinition(id);
        UpdateModel(def, "Definition");
        //UpdateModel(def);
        repository.Save();
        return RedirectToAction("Details", new { id = def.mdID });
    }

Turns out there is a not-so-obvious overload for the UpdateModel method that takes a prefix "Name" property. This is the name of the original encapsulated model class inside the ViewModel. Corrected code fragment from the Controller's Edit Post method is shown above.