This still of programming does not adhere to MVC though, you can't ever swap out the frontend because it's basically merged to the backend and I suspect complex to debug simmilar to JSF.
All MVC (or rather something more modern like MVVM) happens on the server in this scenario, but it doesn't mean that it doesn't happen at all.
A different View + ModelView on top of the same Model could then be used to provide a different frontend if that is ever required (in practice, it probably won't be).
So a pattern is not mandatory with Vaadin, but you can make the UI “simple” changeable with a corresponding pattern of your choice. MVP and MVVM would be good examples of this.
