I haven't looked closely at the Vala stuff that you mentioned, but it is very straightforward to generate shared libraries in Nim with exported functions that you can access via ctypes. That's not quite the same as generating a python module that you can directly import, but it gets the job done.

You might also be interested in NimBorg, https://github.com/micklat/NimBorg , which supports embedding Python or Lua in a Nim program.