Yeah, well, that's the luck of the draw when you only get the chance for one snapshot while someone else is taking the "official photo". I posted it here purely as an example of the image quality from a cheap 1930s folding camera (hence the keywords).
This is from a Nettar? Oh, this is encouraging. I always seem to get vignetting with my post-war Ikonta which isn't real different from the pre-war nettar.
Vignetting is caused by the aperture. The wider you make it, the more vignetting you get. Try stopping down to around f14 and you should have no vignetting problems at all.
Secondly, check that your lens is running at the right speeds. A simple testing device can be had for a couple of dozen dollars. In any case, don't try to use it at the top shutter speed, they are almost always a lot slower than claimed and the strong spring may cause vibrations.
I guess - really not sure - that any vignetting would become more obvious if you are dealing with an under-exposed neg.
A Nettar or Super-Ikonta that is correctly adjusted and correctly used can still produce excellent pictures. They have lots of limitations but if you keep them inside those they're still fine, usable cameras.