In all of the visioneering work I did way back in a previous life, I found that the best people to define the mission and vision were that small group of leaders/champions who would be responsible for inspiring the company and leading it forward.
But I also endorse the idea that as many people as possible should have an opportunity to provide inputs to the decision, and should at least feel like they were involved. It seems a little manipulative in retrospect, but we used to FIRST define the m/v/v with the company/department leadership and THEN run visioneering workshops for all first-level employees, using the "desired outcome" (never revealed to them) as a guiding light for the facilitator.
Those employees were guided through a structured process in which they listed all of the factors that influence their ability to do their jobs, and how those factors were changing, and then described the environment they would be trying to succeed in two years down the line. They would describe how an ideal organization would be structured and organized to best succeed in that environment. Then they would list out the activities they currently engaged in every day, and place them on various grids that identified opportunities for improvement or efficiency. They would then create an optimized grid, compare it with their earlier picture of a successful organization of the future, and develop guidelines for changes beyond simple linear improvements.
What fell out of those sessions was an iterative input to the earlier defined vision and mission statements. It was fed back to the leadership team, who would adjust their definitions accordingly (sometimes not at all, sometimes dramatically). Those final m/v/v definitions were then presented back to the employees in the company or department. Buy-in was virtually guaranteed. The process was usually repeated annually, and became very smooth, collaborative, and efficient.
And an important product of the activity was a performance-improvement needs analysis that the employees themselves had created -- which made the creation and implementation of learning activities a great deal easier.