For me the distinction between a mission and a vision is important if your average employee is going to buy into it. A mission describes your reason for being. But it describes your reason for being TODAY as well as into the future. Your vision is much more future-oriented - it is a description of what your company looks like, how it operates, who it works with and so on at a point in time beyond the present. It is, if you like, the ideal towards which you are working while still staying true to your mission. I have always found it useful to write vision statements in the present tense, as if you are actually in the future, looking around you and describing what you see.
Getting as many people as possible in your company to understand and buy into the vision is important - not least because the common vision helps define and reinforce the common values. The difference between a vision and a hallucination is the number of people who share it.