Purpose: answers the question why? It does not change as we go throughout life. It is not affected by our context or how we perceive it.
Call: answers the questions of what?, where?, and when? It is derived from our purpose. It does change throughout life and if we follow it our context will change. It is all about movement.
Vocation: answers the question of how? Knowing our call and our purpose, our vocation is how we interpret our purpose into our context. As our context changes as a result of our changing call, so our vocation changes.
Its like a song: purpose is the key, call is the rhythm, and vocation is the instrument we choose to play.