We investigated the dynamics and morphology of dewetting of metastable polystyrene films (thickness 75-500 nm) cast on silicon substrates, upon exposure to vapors of a mixture of toluene, a good solvent, and ethanol, a nonsolvent. Adding more than 2 wt % ethanol to the saturated toluene environment resulted in a dramatic increase in dewetting rate, an increase in the contact angle of polymer droplets and hole rims on the substrate, and extensive fingering leading to droplet shedding. Films preannealed close to T-g before exposure to the solvent vapors showed markedly slower dewetting. We conclude that in the presence of a good/nonsolvent mixture the polymer chains transition to a globule conformation, which leads to larger interfacial slip, lower viscosity, and significant elastic stress. The slip length derived in the presence of ethanol vapor is close to the values obtained for polystyrene on hydrophobized silicon. As the dewetting process is so significantly boosted by exposure to mixed toluene/ethanol vapors, polystyrene films as thick as 520 nm could be dewetted.