The word vision brings up feelings of breath-taking vistas and simple clear grand statements. In truth it may be like that in the mind of the vision holder, but ‘getting it out there’ is more difficult. In addition a simple statement is easy to mis-interpret.
A vision is uSDLC parlance is more prosaic. If the vision holders can produce all the ideas that is burning in their brains then this can provide the basis from which a design can be fleshed out.
Think of a vision as a very high level design before it is catalogued and pigeon-holed. When it comes time to clarify and record the high level design, place it on a child page. The raw clarity of the original description continues to have value.