These are what I believe should be the definitions for Mission and Vision statements.
A mission statement is an enduring goal for an organisation or work group, which can be used as a tool for strategic alignment at the highest level. All work should be done in pursuit of that enduring goal.
A vision statement is a presentation of a possible future, an achievable outcome which the CEO or organisational leader believes can and should be achieved. One job of leaders is to look further ahead than others, and set the direction for a group of people towards a goal which they cannot otherwise see being achieved.