  The present work is devoted to the investigation of the interaction between
vortices (topological defects) and site-impurities (structural defects) in the
2D XY model and its influence on the well-known properties of the pure system.
The main goal is a theoretical description of the
Berezinskii-Kosterlitz-Thouless (BKT) temperature reduction by quenched
non-magnetic impurities, based on the vacancy-vortex interactions and the
vortex-pair dissociation mechanism of the transition. The non-magnetic impurity
interaction with a system of vortices can be found either from the
phenomenological theory of topological defects or from the Villain model. We
take both paths and compare the results obtained. Our prediction for the BKT
temperature reduction is confirmed by the available Monte Carlo data.
