I use vignettes all the time when I want to work quickly. No time to waste just the important stuff. It requires I only go for the essence...no messing around with unnecessary stuff.
So what exactly is a "Vignette"? The way I was taught is: The vignette is a small, unfinished watercolor that is simply defined.
Here are some guidelines:
(1) The sketch is small, postcard size 5x7, 6x9 inches.
(2) Sketch does not extend into any of the corners.
(3) The design should touch the sides at least once but need not touch all 4 sides.
(4) The composition is the cruciform.
What is a Cruciform? A cruciform is created when dominate vertical and horizontal shapes intersect. Where these shapes converge it is an ideal place for establishing your center of interest. The intersection is off center both vertically and horizontally. Care should be taken not to have the intersection in the dead center of the paper. The corners are simplified creating negative space.
Here's an example. The photo shows how I eliminate what's not important and simplify the scene.