Writing your vision is your opportunity to think BIG! In fact you can do more than think, you can dream. What will the world (or at least the bit of the world that you'll have an impact on) look like once you - and others like you - have worked your magic?
So if you're a recycling business, your vision might be "a zero-waste economy". Or if you run a crime-reduction initiative, it might be "No crime - and no fear of crime - in our area." Or, to think a bit bigger, if you were running the United Nations, your vision might be "a more peaceful, prosperous and just world."
Think big - when things get difficult it will help to be reminded why you do what you do (and what joins you to other people who share similar visions). A well-defined vision can also inspire other people to join you and help you to work towards your vision.