We analyze the space and time behavior of seismicity at Mt. Vesuvius during the last 20 yr to characterize the seismic regime of the volcano during the present quiescent period. The new results on the volcano structure inferred from active seismic tomography experiments, the newly implemented 10-yr arrival time catalog, and a high-quality digital waveform data set have been analyzed. The back-ground seismicity is concentrated near and beneath the Mt. Vesuvius crater, with depths lying above and below the discontinuity, which marks the transition from the shallow alluvium/volcanic sediments and the Mesozoic carbonate basement. The focal mechanisms of microearthquakes show variable stress-axis orientations as a function of depth, although there is evidence for a clustering around roughly the north-south to vertical directions for the tension axes and east-southeast-west-northwest to vertical directions for the pressure axes. The statistical analysis of the seismic catalog confirmed the tendency of background seismicity to cluster in time, according to a trigger model (as denoted by Vere-Jones and Davies, 1966), that is, the generalized Poisson process. A significant increase of the average seismic energy release with time is observed, which is related to the occurrence of several M_D>3 events in the past 10 yr, accompanied by intense swarm activity. This is consistent with the decrease of the b-value from about 2 to 1 during the same period. The (M_D>3) events are located in the same area and depth range of the whole seismicity, and their fault-plane solutions also show variable stress-axis and nodal-plane orientations. In particular, the moment tensor inversion of P and S waveforms from the largest earthquake in the catalog (M_D 3.6, on 9 October 1999) shows no significant departure from a pure shear, double-couple mechanism, thus suggesting a dominant tectonic-like fracture mechanism. The decrease of parameter b with time is interpreted as a dominant effect of fluid pressure variations on the present seismic regime at Mt. Vesuvius, which could be driven by the progressive cooling of the volcanic system.