Vibrational spectra of ultrathin films of perylene-tetracarboxylic-dianhydride (PTCDA) on the more open Ag(110) surface and the close-packed Ag(111), as recorded by high-resolution electron energy loss spectroscopy (HREELS), are reported. By comparison to density functional calculations of the vibrational frequencies most experimental peaks can be reliably assigned, The internal vibrations of the molecules turn out to be very sensitive towards the effects of the interaction with the substrate. This leads to pronounced differences in the submonolayer spectra from both surfaces. Thus, the results bear some importance in the context of organic epitaxy, which relies on the formation of highly ordered interfaces.