<p>Why not use partial class to build the ViewModel?</p>

<p>Since the definition of viewmodel classes have some definition of the data classes of datamodel, why not try to reduce some work, extending or making partial classes of the datamodel, completing them with command and etc.</p>

In some cases this may be useful, but generally the ViewModel (or PresentationModel) has a different set of concerns that would muddle the rest of the model.
For example, imagine that you have an object model that represents an order processing workflow (everybody's favorite example.) This same model might be underlying a WPF, a web site, and perhaps even other services.
Within the WPF application there will be WPF-specific concerns for presentation. Adding these concerns to primary model (even as partial classes) results in a violation of SRP. Ultimately, it would obscure the intent of model and make it difficult for consumers of the model to know which parts of it where essential and which were incidental.