The measured vacuum energy density entering cosmology is a classical macroscopic quantity. In standard quantum theory, the emergence of classicality from quantum superposition is explained through decoherence: a subsystem loses accessible phase coherence through entanglement with environmental degrees of freedom. This paper applies that same logic to the vacuum itself. An arbitrary finite spherical region of vacuum is treated as an interior quantum subsystem, while its boundary and exterior vacuum degrees of freedom form its environment. Under a holographic assumption, the boundary carries an area-scaling information capacity and functions as the interface through which interior field configurations are encoded relative to the exterior. Tracing over the boundary-exterior environment yields a reduced density matrix for the interior whose off-diagonal components are suppressed by environmental overlap factors. In a Gaussian influence-functional model, the decoherence exponent is controlled by a boundary noise kernel and scales schematically with the number of boundary information cells, Γ_ij ∝ A/ℓ_P², for distinguishable interior configurations. Thus a finite vacuum region cannot be treated as a pristine, perfectly coherent, isolated quantum register. The global vacuum may remain pure, but every finite restriction of it is generically mixed and dynamically decohered by the rest of the vacuum. This vacuum self-decoherence does not by itself calculate the observed cosmological constant. It instead establishes that a finite vacuum region is never operationally described by an unlimited pristine coherent state. This provides a possible physical mechanism underlying the coarse-graining that renormalization implements algebraically.