In this thread above you have pic of Pierburg VNT soenoid:
I did cleaning this way:
Shut small pipe number 2
Stick WD40 (best something that does not leave residue but WD can help) in pipe number 1
Watch as dirt comes out in area of number 3
I must put some disclaimer here:
I am not 100% sure it is correct way of cleaning it, that is just how I did it
So, everything you do, do it on your own risk.
Of course, diagnostic should be done before.
For example: if you remove rubber hose from pipe number 1 when engine is turned off
, and you hear sound of air "coming" into something, than most probably "sponge" in "hidden" pipe number 3 is sticked in position where it is not allowing air to come into rubber hose on pipe number 1.
(And it should if engine is turned off)