Eh, I'm not sure about this.
As mentioned in the comments, by changing mvvm to lang-default, c# will end up being the only tag with a specific language hint, causing every code block (including the XAML) to be highlighted as C#. That's no good.
With lang-xml, a specific language hint, a question with both tags (and thus both language hints) will fall back to lang-default for highlighting. This allows each code block to be highlighted as appropriately as prettify thinks it can.
Personally, I don't think we should leave the question as is, with just the mvvm tag alone. In fact, I went ahead and added the c# and wpf tags to it instead. After all, the language used in the question is C#, while MVVM is merely a pattern. It was probably given the lang-xml hint based on how MVVM is most prevalent in WPF/Silverlight application development, which uses XAML for views.