In this talk, I will present how solutions to Einstein-Vlasov equations can be approximated by solutions to Einstein vacuum equations. I will start with the geometrical optic construction of Choquet-Bruhat in 1969, revisited by Touati in 2022. I will then explain how microlocal deffect measures can be used to prove the Burnett's conjecture. This conjecture of 1989 says that solutions to Einstein vacuum equations converging weekly in W^{1,\infty} have a limit which satisfies Einstein-Vlasov equations. This is a joint work with Jonathan Luk.