The observation of photon cascade emission in Pr3+ activated LaMgB5O10 under 185 nm vacuum ultraviolet excitation is reported and discussed. In this host lattice, the lowest energy state of the Pr3+ 4f5d excited configuration is located above the S-1(0) state. The excitation of the 4f5d state by vacuum ultraviolet photons results in radiative emission originating from the S-1(0), level state. The absence of emission from the P-3(0) state is attributed to the efficient multiphonon P-3(0)-->D-1(2) relaxation in this lattice.