Just a side note. Learning C++ and learning VTK should be two different goals.
Learning VTK from C++ or Python should mostly be the same as they both call VTK/C++ API.
The big difference, is that by starting with Python, you will skip all the C++ compilation hurdle that is not specific to VTK. But you will still be able to understand VTK logic and how to leverage it.
So bundling C++ & VTK in your learning path is definitely going to be though.