Do you get spheres elsewhere? I mean, try to run the code in another hardware. It is also possible that it is a graphics card driver bug. So, make sure you have the latest version installed.
Unfortunately, QVTKRenderWindowInteractor doesn’t seem to have a way to set surface format (if it is using the OpenGL backend) which would be a solution according to QVTKOpenGLWidget: Vertices disappear when RenderPointsAsSpheresON().
Perhaps you could take this issue to developers of QVTKRenderWindowInteractor, since there is a solution to your issue in the C++ API.