This paper presents a mechanism for the emergence of an effective cosmological constant based on the spectral structure of a vacuum defined over a discrete causal geometry. Within the Factorization by Function Zeros (FZF) formalism, fundamental spacetime is described by a $(1+1)$-dimensional Lorentzian lattice, where admissible states are organized by a discrete spectral operator associated with arithmetic integrality constraints. Key Findings: Arithmetic Spectral Gap: The paper establishes the existence of a universal, irreducible minimum spectral gap ($\Delta = 4$) in the off-shell regime. This gap is derived purely from modular congruence on the causal lattice ($L_N \equiv 0 \pmod 4$). Causal Dilution Mechanism: The observable cosmological constant ($\Lambda_{\mathrm{eff}}$) emerges not as a fundamental parameter, but as the effective density of this residual spectral cost diluted over a finite causal window. The volume of this window grows quadratically with the causal horizon ($|W| \sim B^2$). Solution to the Hierarchy Problem: The model naturally explains the magnitude hierarchy of $10^{-122}$ in Planck units. This is achieved by the inverse scaling of the vacuum energy with the accessible causal volume, avoiding the need for fine-tuning or divergent mode summations common in Quantum Field Theory. Thermodynamic Stability: The stability of the spectral vacuum is justified by identifying the resolution parameter $\beta$ with the inverse of the cosmological horizon temperature (Gibbs-Hawking), implying a thermodynamic freezing into the fundamental gap state. Matter-Vacuum Phase Transition: The presence of on-shell states (matter) triggers the collapse of the free energy, suggesting that matter and vacuum correspond to distinct phases of the same discrete spectral system, separated by arithmetic admissibility conditions. Methodology: The study utilizes a pre-particle formalism on a discrete Lorentzian lattice. It employs analytical number theory (Weyl's criterion, Van der Corput method) to verify the statistical robustness of the spectral background and provides numerical simulations to corroborate the stability of the spectral plateau. Keywords: Cosmological Constant, Discrete Spacetime, Spectral Gap, Vacuum Energy, Causal Sets, Emergent Gravity, Number Theory.