DTO = ViewModel?

The canonical definition of a DTO is the data shape of an object without any behavior.

ViewModels are the model of the view. ViewModels typically are full or partial data from one or more objects (or DTOs) plus any additional members specific to the view's behavior (methods that can be executed by the view, properties to indicate how toggle view elements etc...). You can look at the viewmodel as all the data for a view plus behaviors. ViewModels may or may not map one to one to business objects or DTOs.

By the way, NHibernate projections come in handy if a certain viewmodel needs a subset of the data from a persisted object.

ViewModel in ASP.NET MVC practice is the same as the DTO, however ViewModel in MVVM pattern is different from DTO because ViewModel in MVVM has behaviors but DTO does not have.

DTO != ViewModel

In the MVVM pattern the ViewModel is used to isolate the Model from the View. To represent the Model you could use simple DTO classes, which again is mapped to a database through e.g. NHibernate. But I've never seen a ViewModel class which is modelled as a DTO.. ViewModel classes mostly have behavior, which DTOs don't have.