Melt inclusions in Ca-pyroxenes from pyroclasts of several Vesuvius eruptions show that pyroxenes (Wo46–48-En45–50 to Wo50–52-En20–25) formed in shallow reservoirs from K-basalt and/or K-tephrite to K-phonolite magmas over a wide temperature range (800–1200 °C). The data set reflects the general modalities of magma evolution at Vesuvius, where the highly evolved compositions are generally reached under closed-conduit conditions, forcing magmatic crystallization under increasing activity of dissolved volatiles. These data allow the establishment of two empirical geothermometric laws that link temperature either to the equilibrium constant of melt-pyroxene pairs or to a compositional parameter of the melt (e.g., CaO). They are used to reconstruct the thermal and compositional evolution of Vesuvius magmas. This evolution is often affected by syneruptive mixing processes in the magma storage zone that alter preeruptive conditions. The method can have general applicability for the definition of empirical thermometric laws.