im sure that someone can correct me if Im wrong, but vignetting is part of the noctilux magic... it read in books and other experts countless times that a summilux 1.4 is brighter than a noctilux 1.0, since although the notci is 1.0, is only in the center, as soon the circle enlarges through the edges it starts to loose stops as much as 1,5 to 2,0 stops; therefore the summilux with its constant 1.4 aperture is more brighter in fact.
Although could be perceived as a bad thing, its one of the characteristics that contribute to the uber gorgeous bokeh of the noctilux and its signature, its always mentioned as well.
Thats the noctilux - either you love it or you hate it.
So the filters may be not the culprit always, since the lens can vignete hard per se.
Take my advice with a grain of salt, just used one for a couple of hours ( enough to get me thinking until today )