I've seen vision documents that were 50 pages long, carefully formatted with research, diagrams, and strategic thinking. I've also seen visions that were a couple of pages of bulleted items, with a few sentences describing each one. Depending on the project, different amounts of structure and planning are needed. Don't make the mistake of thinking that planning documents are fixed, rigid things: they're just documents. How deep or fancy they need to be depends on the nature of the project and the culture of the team. However, good vision documents tend to cover the same kinds of questions, but the material varies in depth and rigor.
To help you figure out how much structure and investment your vision document needs, consider the following questions:
How many different people will be impacted by the project? How many different organizations are they in? How will you set expectations up, down, and across each organization properly?
How many valid questions does the team itself have about the future? How much do people need to know about what they'll be doing and why they'll be doing it?
What depth of feedback on project direction do you want from others?
How much explaining of decisions do you want to have to do in person? (A good vision should stand on its own in representing the project to many people.)
How much depth of knowledge and thought should a project leader provide to the organization as part of making project-level decisions? (A vision provides ...