Abstract: This paper introduces the "Superfluid Manifold" framework, a unified field model that treats the vacuum of space not as a static geometric background, but as a physical, thermodynamic medium. By applying the principles of fluid dynamics to the vacuum, we provide a deterministic solution to the Pioneer Deceleration Anomaly and reconcile the geometric formalism of General Relativity with observed quantum-limit phenomena. Key Contributions: The Schoenfelder Index (n_{\rho}): Introduction of a local refractive measure of vacuum permittivity, defining how the "stiffness" of the manifold varies with energy density. Vacuum Phase Transitions: A model for the state-changes of spacetime (Solid, Liquid, Gas) governed by energy density thresholds (\Phi_{crit}), converging at the GZK and Schwinger limits. Leidenfrost Displacement Field: A theoretical derivation for propulsion and movement within a superfluid medium, suggesting a pathway for non-ballistic field-interaction technologies. Refractive Gravity: A reinterpretation of gravitational acceleration as an emergent hydrostatic pressure gradient (\nabla P) within the manifold. Context: This research transitions physics from a geometric-only perspective to an "Ontological Realist" model, providing a stable theoretical foundation for understanding the vacuum as a measurable, interactive substance.