ModuleNotFoundError: No module named 'vmtk'

Hello all,
First of all, I would like to thank you for developing Slicer, I am a new user and it seems to be a powerful framework.
I have a Python project, PP, for image processing relaying on VTK, SimpleITK & VMTK.
I would like to develop a Slicer extension module, SEM, encapsulating some scripts from PP.
I was able to implement the modules widget, and now I have to code the SEM Logic, which consists of calls to function on scripts on PP.

I have a symbolic link to my PP folder in the SEM folder. This allows me to import my PP scripts.
I am having troubles importing scripts from PP which depends on VMTK, for example, let’s say that I want to import “PP/” into my SEM module code

# start
import PP.scriptA
# module content...
# end

# start --------------------------------
import vmtk.vmtkscripts
# script content...
# end ---------------------------------

Loading the module in Slicer 4.11.20200930 r29402 / 002be18, which has the SlicerVMTK extension results in the following error.
ModuleNotFoundError: No module named ‘vmtk’

I assume that in order to use any vmtk functionality I needed to install the SlicerVMTK extension, but I am starting to think that maybe that is not the way to use vmtk from my SEM extension.

Could someone point me in the right direction?