For MVVM (unlike MVP and the like), you need the "glue" that ties viewmodels to views. Rolling it out by hand is far from trivial, not to mention things such as IDE support etc.

The official Data Binding library by Google - in my opinion, the only sane way of attempting MVVM on Android - is fairly new. If I'm not mistaken, they've only recently moved it out of preview stage, and they didn't tout about it much anyway.