The effects of supporting substrate and thermal annealing on plasticization of poly(vinyl acetate) ultrathin films by absorption of ambient moisture have been studied by local dielectric spectroscopy. Upon exposure to ambient moisture, the relaxation rate of the alpha-process increases to a different extent at variance of film thicknesses and supporting substrates. Namely, on hydrophobic gold substrates the speeding up is slightly reduced by decrease of film thickness down to about 20 nm. Moreover, the increase of the relaxation rate measured on a 21 nm thick film supported on gold is smaller compared to that measured on a film with similar thickness supported on the relatively hydrophilic aluminum. We interpret such results in terms of the presence of an interfacial polymer layer at the substrate surface with different contents of water in dependence of the substrate. Furthermore, plasticization effect in ultrathin films significantly decreases after annealing at high temperature (similar to T-g + 60 K) and for times longer than usual time scales of relaxation processes. Such results are consistent with the formation of a high-density irreversibly adsorbed polymer layer at the interface, able to hinder the effect of the substrate.