Well, as I see it, the best vignettes share a few features:
Use of space - Since vignettes sit on a small footprint, it is important for them to utilize the available space wisely. Often this involves the vertical dimension, as shown in The Great Escape by Graviton. This scene has details at three different levels, from the underground tunnel to the guard atop the wall.
Action - These shouldn't be static creations, but should instead have a sense of movement. In this Peter Pan scene by Tshirane, it really feels as if the characters are flying.
Storytelling - Many of the best vignettes tell a story. Found at Last by Cedarsith sets up the storyline with no need for explanation.
Sense of humor - Finally, my favorite vignettes tend to have a sense of humor. Last Halloween, for instance, Nathan Wells posted Trick or Eek.