The piloting VESUVIO experiments RB820478, RB610497 on (NH4)2PdCl6 and (NH4)2TeCl6 and RB 10067 on (ND4)2PdCl6 have shown that: (i) the H/D dynamics in NH4+/ND4+ is largely decoupled from PdCl6-/TeCl6 at 5 - 300K, suggesting that ammonium behave like pseudospherical ions embedded in the sublattice of acthahedral PdCl6-/TeCl6 sites, (ii) H momentum distribution in NH4+/ND4+is non-Gaussian. A classical onset exists for both compounds: at 50K both undergo a transition from quantum tunnelling to stochastic motion despite tunnelling frequencies in both substances differing three orders of magnitude. We will have a closer look at this classical onset of H/D dynamics by measuring vibrational density of states (VDOS) across the 'classical onset' at 30, 40, 50, 60 and 70 K and 300K. Ab intio PW-DFT (CASTEP) will be perfromed to simulate the VDOS and compare/contrast it with the obtained data.