The vignette filter is intended for adding artistic vignetting to your image. This vignetting filter is placed relative to the crop, if cropping is used.
For correcting vignetting caused by the lens light fall-off (as opposed to this filter which is not for correction but for artistic effect), use the Vignetting Correction filter in the Transform tab, in the Lens/Geometry tool. Even better, use the Flat Field tool.
The amount of darkening the filter will apply, in stops. The full strength is reached in the corners of the image. If you apply a negative amount the corners will be brightened instead of darkened.
The feather slider controls the width of the feathering. If at 0 only the corners will be feathered and the rest of the image will not be affected by the filter. At 50 the feather reaches halfway to center and the rest is unaffected, and at 100 the feather reaches all the way into the center.
The roundness slider controls the geometry of the filter. At 0 the shape is rectangular (with rounded corners), at 50 it is a fitted ellipse, and at 100 it’s circular. Note that if your image is square the fitted ellipse will of course be a circle, so the shape will then not change in the range 50 to 100.