The current version Gimp 2.10 uses a GEGL (see http://gegl.org/) operation for creating a vignette. The page for describing the feature is http://gegl.org/operations/gegl-vignette.html and bottom of that page, a link to the code.
In earlier versions of Gimp a vignette effect can be 'hand-made' usually by filling a fuzzy selection or application of a gradient in a layermask.
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In earlier versions of Gimp a vignette effect can be 'hand-made' usually by filling a fuzzy selection or application of a gradient in a layermask.
You might get more information by subscribing to one of the mailing lists https://www.gimp.org/mail_lists.html