While reliable experimental positron lifetimes exist for mono- and divacancies in silicon, up to now no agreement has been reached on the sizes of vacancy clusters detected in irradiated and partly annealed silicon. Hence, using a density-functional based Tight-Binding method, we investigate the stability of different vacancy clusters (up to 17 vacancies). For direct comparison to experiment we compute the positron lifetimes for the most stable structures improving the atomic superposition method by including the influence of the trapped positron on the ionic structure. These results give a consistent picture of stable vacancy clusters in Si, in agreement with EPR results.