We consider the Br vacancy in CsPbBr₃ as a prototype for the impact of lattice dynamics on defect energetics in halide perovskites (HaPs). Using first-principles molecular dynamics based on density functional theory, we find that the static picture of defect energetics breaks down; the energy of the V_Br level is found to be dynamic, oscillating by as much as 1 eV on the ps time scale at room temperature. These significant energy fluctuations are correlated with the distance between the neighboring Pb atoms across the vacancy and with the electrostatic potential at their atomic sites. The unusually strong coupling of lattice dynamics and defect energetics bears important implications for both experimental and theoretical analysis of defect characteristics in HaPs and may hold significant ramifications for carrier transport and defect tolerance in this class of photovoltaics.