Graphene thermionic emission involves energetic electrons residing far away from the Dirac point. Yet, the simple Dirac approximation, accurate only for electron energy < 1 eV, is often used in the modeling of graphene thermionic emission devices. In this work, we construct an emission model using the full-band tight-binding model of graphene. We show that the simple Dirac model breaks down for vacuum thermionic emission. The Dirac model severely overestimate the power density by ~50% in comparison with the full-band model.