Interlayer phase coherence in the vortex state of Bi₂Sr₂CaCu₂O_8+δ is studied by Josephson plasma resonanceover a wide field range. In almost the entire region of the vortex liquid phase, the plasmon frequency is well described by the high temperature expansion theory that assumes almost decoupled layers. The plasmon frequency changes drastically below the melting transition line at low field. The rapid decrease of the interlayer coherence with decreasing T below the irreversibility line reported previously is shown to be due to the unusual critical state of this material.