We teach ScrumMasters and Product Owners in training how to write product vision statements as part of the extended planning framework of Scrum/Agile. There are a number of ways this can be achieved - elevator statement [Moore], Design-the-box exercise [Highsmith]. Some participants have mentioned that their organizations write "Vision Documents" that go one level deeper than a mere statement. The culminations of these exercises are black-and-white, resulting in a statement, a document, a box.
In thinking about product vision this evening, and on the 'softer side', I like to think that the product vision should have a motivating or inspirational aspect.