From my brief experience testing the 'Bokeh Master's Kit' I learnt these things:-

a) As you probably know a wider camera aperture allows the shaped background blur to be more pronounced. On the opposite side, a smaller aperture will not only reduce the bokeh shaping but increases the vignetting *darkness at the edges)

b) A longer focal length on the lens with the wider aperture helps increase the shaped bokeh effect. As well as this, having a longer focal length (50mm in your case) will cause less vignetting as the lens is looking at less of the cut-out and more of the view through it. So using it at the 18mm end of the lends would not only decrease any shaped bokeh effect, but also cause more dark edges

c maybe) Longer exposures may cause more vignetting to be visable as the lighter areas are exposed more and the darker ones less so causing a greater difference between. This may have more of an effect on film that digital sensors but that's purely my speculation, and any difference I saw when testing was more likely because of the small apertures I used to get that longer exposure time.

So in summary, widest apertures, longer focal lengths and maybe less exposure time should reduce any vignetting you get on photographs as well as make the shaped bokeh more pronounced. I know images without vignetting can be achieved as I took test videos too and there was none on there (taken on a manual 50mm f1.8).

It's amazing how small the cut-out hole can be in front of a lens without it blocking out any image