If you asked each member of your team why they were working on what they are currently working on, what would they say? Could they even give you a coherent answer? If they did, would their answers all agree?
This is a companion discussion topic for the original blog entry at: http://www.codinghorror.com/blog/2005/08/vision-quest.html
I typically look to the stakeholder needs to get this information; however, often times this is not conveyed to the development team. Or it is forgotten altogether. And rarely is it boiled down to a vision statement.
Vision statements (and mission statements) focus you on the task and help guide you in making decisions. It’s really unfortunate they are so misused to the point of becoming lame.