Yeah, there is a small filter on the pressure converter and I guess it could be blocked by the exhaust soot - not 100% sure how this would affect it's operation but I guess it could well be compromised by a dirty filter - internet "evidence" suggests this is the case.
Slightly different pressure converter model but you can see the filter, second item from left.
(Image from this thread - which is interesting...
https://www.e90post.com/forums/showthread.php?t=1589166
The vacuum reserviour is simply to provide a near-constant vacuum "pressure" (negative pressure) for the pressure converter to work with, since it opens and closes very rapidly to modulate the VGT actuator position. Without this reservoir, the pressure converter is working with a vaiable vacuum level, meaning the VGT operation is dependant on vacuum level (via RPM).