anyway, I still don’t understand what kind of parameters all those vtk-methods expect (apart from type) and what they are needed for. I would like to understand why the internal parameters (diameter, alpha, beta,…) are calculated like that. Is there something like a detailed example or an user documentation?
Yes, this exactly the main problem with VMTK: there is nearly zero API documentation. You can ask about the parameters on the VMTK mailing list then write what you learn to documentation of the corresponding class in VMTK (they accept pull requests).