We study a multiorbital model for the alkaline iron selenides K1−xFe2−ySe2 using a slave-spin method. With or without ordered vacancies, we identify a metal-to-Mott-insulator transition at the commensurate filling of six 3d electrons per iron ion. For Hund’s couplings beyond a threshold value, this occurs via an intermediate orbital-selective Mott phase, in which the 3d xy orbital is Mott localized while the other 3d orbitals remain itinerant. This phase is still stabilized over a range of carrier dopings. Our results lead to an overall phase diagram for the alkaline iron selenides, which provides a unified framework to understand the interplay between the strength of the vacancy order and carrier doping. In this phase diagram, the orbital-selective Mott phase provides a natural link between the superconducting K1−xFe2−ySe2 and its Mott-insulating parent compound.