We present the Vacuum Saturation Model (VSM), a phenomenological framework in which dark energy and dark matter originate from a single mechanism: the continuous transfer of energy from radiation to the vacuum. The vacuum possesses a finite energy capacity; energy deposited below saturation accumulates as dark energy, while excess energy in saturated regions precipitates as cold dark matter. We specify modified Friedmann and continuity equations with an environment-dependent coupling function Γ(z) that is naturally inactive during Big Bang Nucleosynthesis (suppression factor ~10⁻²³) and does not alter the CMB acoustic peaks (sound horizon deviation -1 strictly, with w ≈ -0.99 constant across redshift—is currently supported at up to 4.2σ by DESI DR2. If w = -1 is confirmed to high precision by the Vera C. Rubin Observatory or the Nancy Grace Roman Space Telescope, the model is falsified. A candidate Lagrangian of the form L_int = M⁻²φF_μνF^μν is proposed for future investigation.