House on Mango Street is made up entirely of vignettes. A vignette is a little slice of life or snapshot of experience. In French, it meant a small portrait -- a painting. Students may have heard examples of vignettes or learned the elements of autobiographical incident.
One of these actvities, Snapshot of Experience, asks them to bring a photo that is tied to a vivid memory and write about it. They can now write a short vignette associated with their memory or photo. A vignette is very specific, as with a visual portrait, using significant details. But in a vignette the writer describes or portrays features or traits in simple, vivid ways. A vignette uses more straightforward "showing" with words rather than "telling," less commentary and abstraction, and it lets the meaning reside within the image or experience itself, open to various interpretations.
So, one evaluating criterion for the quality of writing in a vignette may be whether or not the images evoke an intended response in the reading audience. But then again, another might be whether or not it triggers unpredicted and unpredictable emotional response.