Vacuum Energy from a Unique Closed Variational Structure

The divergence of vacuum energy in quantum field theory is typically interpreted as a mismatch between theoretical predictions and cosmological observations. In this work, the problem is reformulated at the level of the admissible configuration space. We construct the class of admissible variational functionals defined by minimal structural conditions, including variational closure, spectral stability, and internal consistency. It is shown that this class admits a unique representative, given by a closed quartic functional. Within this framework, physically admissible configurations are not assumed but derived as those satisfying a spectral stability condition on the second variation. This defines a restricted domain of configurations over which physical quantities must be evaluated. As a consequence, vacuum energy, when defined over the admissible domain, is finite. The apparent divergence arises from extending the calculation to configurations that do not correspond to spectrally stable states. The result does not rely on model-dependent assumptions or cancellation mechanisms, but follows from the internal structure of the admissible variational class. In this sense, the vacuum catastrophe is resolved by identifying the correct domain of physical configurations rather than modifying the underlying dynamics.