ViewModel Best Practices

From this question, it looks like it makes sense to have a controller create a ViewModel that more accurately reflects the model that the view is trying to display. However, I'm curious about some of the conventions.

Basically, I had the following questions:

1. I, normally, like to have one class/file. Does this (preference) make sense with a ViewModel if it is only being created to hand off data, from a controller to a view?
2. If a ViewModel does belong in its own file, and you're using a directory/project structure to keep things separate, where does the ViewModel file belong? In the Controllers directory?

That's, basically, it for now. I might have a few more questions coming up, but this has been bothering me for the last hour, or so; and I can't seem to find consistent guidance, elsewhere.

EDIT: Looking at the sample NerdDinner app on CodePlex, it looks like the ViewModels are part of the Controllers, but it still makes me uncomfortable that they aren't in their own files.

I create what I call a "ViewModel" for each view. I put them in a folder called ViewModels in my MVC Web project. I name them after the controller and action (or view) they represent. So if I need to pass data to the SignUp view on the Membership controller I create a MembershipSignUpViewModel.cs class and put it in the ViewModels folder.

Then I add the necessary properties and methods to facilitate the transfer of data from the controller to the view. I use the Automapper to get from my ViewModel to the Domain Model and back again if necessary.

This also works well for composite ViewModels that contain properties that are of the type of other ViewModels. For instance if you have 5 widgets on the index page in the membership controller, and you created a ViewModel for each partial view - how do you pass the data from the Index action to the partials? You add a property to the MembershipIndexViewModel of type MyPartialViewModel and when rendering the partial you would pass in Model.MyPartialViewModel.

Doing it this way allows you to adjust the partial ViewModel properties without having to change the Index view at all. It still just passes in Model.MyPartialViewModel so there is less of a chance that you will have to go through the whole chain of partials to fix something when all you're doing is adding a property to the partial ViewModel.

I will also add the namespace "MyProject.Web.ViewModels" to the web.config so as to allow me to reference them in any view without ever adding an explicit import statement on each view. Just makes it a little cleaner.