In this post Michael Washington explains how to place your web service methods in your Model and simply call them from your ViewModel using RX Extensions.
With “Simplified MVVM” you can simply place your web service methods in your Model. The problem you run into, is how do you make an asynchronous web service call and fill a collection in your ViewModel? One method I have employed in the past is to pass an instance of the ViewModel to the Model, however, the problem this causes, is that you have now tightly coupled your ViewModel and your Model. It is also difficult to consume your Model from multiple ViewModels when you do it this way.