As stated in the previous response by Kiran, a Mission statement should reflect the purpose and objectives of the institution. It needs to be congruent with the mission and values of the larger/governing institution or body. Basically, it is "why" you exist.
A Vision statement is about the future. What are your aspirations? It is "where " you want to go and "what' you want to be in the future. It should be written with enthusiasm.
Both statements need to be clear and concise.
When working on a similar project regarding mission and vision statements, I find it helpful to review the statements of institutions similar to ours. It is also helpful - and fun - to look at those of corporations. There are many styles out there.