"Corporate mission" is another way of describing a company's management philosophy: a company's long-term values and raison d'etre that do not change easily because of top management shifts or market changes. Corporate mission establishes a clear long-term goal for a company for all employees to aspire.
In contrast, a "corporate vision" concretely describes how a company sees itself in the future. In the current age of rapid change, a corporate vision is of a more medium-term nature.