Q:

ASP.NET MVC 3 POST ModelState and ViewModel

In my ASP.NET MVC 3 app I have a page where, after its form is submitted I need to change a value from the form. If I do it in the ViewModel it has no effect, I have to do it use ModelState["Field"].Value.
The thing is that I actually have a lot of work I have to do on the ViewModel when the pages loads AND after the POST. So I have a method which receives the ViewModel and do all that work. But, as I said, after the POST, modifying the ViewModel object is useless. Why is that? The method on the controller is returning View(viewModel);. Shouldn't the view use the ViewModel values?
I'd like to understand why is useless to modify the ViewModel and also know if there is some method in the ModelState which "refresh" the fields values throw a ViewModel object.
Here is my controllers code:
[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Index(MyViewModel viewModel)
{
    try
    {
        if (ModelState.IsValid)
            //Do stuff and redirect
    }
    catch(Exception ex)
    {
        //Log and add error to the ModelState
    }

    someMethodWhichDoesALotOfThingsInTheViewModel(viewModel);
    return View(viewModel);
}

I think the rest of the code is unnecessary, if anyone think it would help let me know!
Update
I don't want to lose any errors, not even those of the fields changed in the ViewModel. A good example would be a captcha, if the users makes any error in the form (only with the captcha field or only with any other fields and not with the captcha) I want to show him all his errors and also update the captcha. It makes no sense to render the form with the old captcha value inserted so I want to blank that field.
Update 2
I've opted to put @Value = Model.PropertyInViewModel in the HTML Attributes of each Html.TextBoxFor. But I'd prefer to avoid that nasty work so if someone comes out with any better solution please let me know. 

A:

The following will work:
ModelState.Remove("MyProperty");
viewModel.MyProperty = "new value";

You need to remove it from the model state because HTML helpers first look in there when binding their value.

A:

All I could came out with is to add @Value = Model.PropertyInViewModel to each Html.TextBoxFor that is going to be modified in the ViewModel.
Another way, as @Darin Dimitrov said, would be to make my own helpers.

