i think it is actually sometimes just an actual lens( something about the way the elements are can make it vignette, i think that is probably worse with relatively cheaper lenses). also using some filters ( like a graduated nd filter) can cause it on wider angles.( like with cokin filters you need to buy a wide angle holder) and sometimes just the way the light hits the hood or filter can cause it. if you were using the right hood and don't usually have the problem with that lens i would guess it was the filter. i think sometimes you get a natural vignette looking area in really wide angles but that wouldn't be the case here. i think it's the aperture so not sure how the 135 could affect it. most of what i have read about it seems to be with a wider angle rather than a narrow angle and they tell you to stop down a bit to help get rid of it. do you remember if they were they all taken same equipment ?
you might be able to fix it in lightroom if you use that. there is a place for lens distortion. i actually like a light vignette to kind of easy out the edges and point toward the middle although that might be a little to dark for that.
