Monte Carlo simulations combined with the embedded-atom method potential have been employed to investigate the microscopic process of the S5 tilt grain boundary sliding in aluminum. We have studied the atomic structures and the grain boundary sliding/migration energy profile at elevated temperatures in the absence or presence of vacancies. The annealing temperature is found to play an important role in determining the grain boundary energetics and mobility. Contrary to "static" simulations, the simulated annealing (SA) produces new lower energy states of the complex and low-symmetry grain boundary structure. The vacancy formation energy at the first layer from the interface is found to be significantly lower than that at the other layers and the bulk. On the other hand, the vacancy at the interface has a significantly higher formation energy compared to bulk, in very good agreement with recent ab initio electronic-structure calculations. For both "static" and SA simulations, the grain boundary sliding energy profile is smooth, exhibiting several energy peaks and valleys, where the latter are associated with grain boundary migration. The SA scheme reduces the grain boundary sliding/migration energy barrier by about a factor of 3 and increases the rate of migrations. The distribution of atomic energies helps identify the atoms that play a key role in the grain boundary sliding and migration. The grain boundary sliding energy profile in the presence of a vacancy placed at the first layer is very similar to that of the clean boundary, while the vacancy at the interface increases the grain boundary energy and leads to no migration.