  Global fits explore different parameter regions of a given model and apply
constraints obtained at many energy scales. This makes it challenging to
perform global fits of simplified models, which may not be valid at high
energies. In this study, we derive a unitarity bound for a simplified vector
dark matter model with an $s$-channel vector mediator, and apply it to global
fits of this model with \GB in order to correctly interpret missing energy
searches at the LHC. Two parameter space regions emerge as consistent with all
experimental constraints, corresponding to different annihilation modes of the
dark matter. We show that although these models are subject to strong validity
constraints, they are currently most strongly constrained by measurements less
sensitive to the high-energy behaviour of the theory. Understanding when these
models cannot be consistently studied will become increasingly relevant as they
are applied to LHC Run 3 data.
