Behavior of an argonlike cluster in supersaturated vapor at constant temperature and pressure is simulated. In contrast to the results of conventional microcanonical ensemble simulations, instability of cluster size is observed (unlimited growth or complete evaporation). This defines cluster critical size, which is noticeably larger than that predicted by the classical nucleation theory. Analysis of cluster collective vibrational spectra shows that its evaporation is intimately connected with spontaneous emergence of the lowest modes of breathing vibrations. The cluster phonon spectrum resembles that of a strange attractor.