Find a topic to write about. A short vignette needs to be a personal and meditative thought that pops up in your mind. It needs to be something that means a lot to you, something that comes from your heart. The inspiration to write a vignette usually comes from a need to express yourself and your thoughts in some other form.
Draw an association diagram. To help organize your thoughts and ideas, and really turn it into something you can write about, use an association diagram. Place your topic in the center and surround it with associated words. Say you have decided to write about winter. You might associate winter with the snow, Christmas, frosty windows and ice-skating. Connect all of these words to your central word and each other in a way you see fit. Take a few minutes to establish relationships among the words you compiled. Your vignette will flourish with these details.
Write your short vignette. Close your eyes, relax and think about the words you have on the page. Try to feel the emotions and energy that these words conjure up inside you. When you feel ready, write what you feel now. Refine later.
Elaborate on your thought. Once a thought has really gathered in your head, it becomes easier to write spontaneously. Write until you have exhausted all of the material that your topic gives you. Refine grammatical errors, spelling and word choice later.
Let it breathe. After you feel there is nothing left to say, leave your vignette alone for a while. Give it a night. When you feel like it, go back, re-read it and edit it. There is no argument or persuasion in a vignette. As long as it captures the mood as you felt it, the vignette has served its purpose. You have a slice of your life in a short and meaningful passage.