I want to know the answer to the following question:
Who's equations  helped to imply vanishing of the Ricci tensor?
All the background I'm provided with is that "Vacuum does not mean that the curvature of space-time is necessarily flat: the gravitational field can still produce curvature in a vacuum in the form of tidal forces and gravitational waves (technically, these phenomena are the components of the Weyl tensor).". Is that enough to answer the question?
Answer: no