It depends on your experience.
I personally felt that this was a lacking area in MVVM discussions, which was what led me to write my series on MVVM and WPF from a Windows Forms developer perspective.
I would not try to learn WPF without learning MVVM. You'll end up trying to "unlearn" many aspects, as WPF really doesn't make sense, completely, until you understand the separation of presentation and logic that's pervasive throughout the architecture. MVVM is really just extending this same separation into the application space.
As for "MVVM in Windows Forms" - a lot of people try to do this, but it's not really MVVM. In order to do "MVVM" you need access to a richer form of data binding than Windows Forms provides. "MVVM in Windows Forms" is really, typically, just Model-View-Presenter with a different name (riding the MVVM bandwagon, I'd guess), but not really the same as MVVM in WPF.