Using a wave packet simulation on simplified potential curves, we investigate the vibrational dynamics of NaI in its first excited electronic state, both isolated and aggregated with a molecule of acetonitrile. The probe signal is obtained by resonant multiphoton ionization (REMPI) of the diatomic chromophore NaI via Rydberg states. The addition of acetonitrile is found to change greatly the ionization signal, suggesting strong modification of the reaction dynamics.