Temporal evolution of vibrational energy distribution during vibrational cooling and heating in a molecular cluster is simulated using techniques of molecular dynamics. The cluster is composed of hundreds of naphthalene molecules with fused atoms for C-H groups. In vibrational cooling, a hot molecule (300 K) is allowed to cool in a cold cluster (10 K). In vibrational heating, a cold molecule (60 K) is heated in a hot cluster (300 K). Normal vibrations of the model molecule are analyzed, and instantaneous amplitudes of the vibrational modes are calculated from trajectories. Vibrational energy transfer processes are discussed. In vibrational cooling, the out-of-plane vibrations play important roles in intramolecular transfer processes and the optically active in-plane vibrations are mainly involved in intermolecular resonant transfer processes. Intramolecular transfer processes through out-of-plane vibrations are significant in vibrational heating.