  The question of whether layer decoupling and vortex-lattice melting occur
seperately or not in the mixed phase of pristine layered superconductors in the
extreme type-II limit is studied through a partial duality analysis of the
layered XY model with uniform frustration. We find that both transitions occur
simultaneously if the normal/superconducting transition of the vortex lattice
in an isolated layer is first-order and if a sufficient degree of layer
anisotropy exists. We also find that a cross-over to a highly entangled vortex
lattice phase with relatively low phase rigidity across layers does not occur
in practice under any circumstances at temperatures below the two-dimensional
vortex-lattice melting transition.
