"I cant find anything in-camera that turns vignetting on or off (i have d3100)"
There is NO WAY to turn vignetting on or off - it is only during post processing possible to compensate for it (artificially to lighten the darkened corners of the image). Best software for it is Photoshop (jpgs being correctable too). http://goo.gl/aJl3I9
Does this mean jpegs made by the camera won't have vignetting corrected either?