Ernest Watson shows a nice trick for vignetting a pencil sketch.
He allows the outer edges of the drawing to gradually dissolve into simple construction lines. The windows are nicely varied, too. No two are alike.
This 1916 drawing is from Arthur Guptill’s book “Sketching and Rendering in Pencil" (republished as "Drawing and Sketching in Pencil,” available here in book form and here for free download.