This paper presents a comprehensive theoretical framework deriving gravity, inertia, and the nature of matter from hydrodynamic first principles. Starting from two foundational axioms—(1) motion is purely relative with no experimentally accessible absolute frame, and (2) oscillation traps energy into stable configurations—we demonstrate that known physics emerges as low-energy effective field theory describing collective behaviour of a substantive vacuum medium. The framework synthesises Superfluid Vacuum Theory (Volovik), Stochastic Electrodynamics (Haisch-Rueda-Puthoff), and Sakharov's induced gravity to derive: emergent Lorentz invariance from Fermi point topology; matter as self-sustaining oscillons maintained by ponderomotive forces; gravity as the Secondary Bjerknes Force (acoustic radiation pressure between resonant structures); and the Equivalence Principle as tautological rather than coincidental. Key results include resolution of the vacuum catastrophe via Gibbs-Duhem equilibrium (with P_vac = 0 holding regardless of scale), explanation of the hierarchy problem (why gravity is 10³⁶ times weaker than electromagnetism), and alternative interpretation of dark matter as vacuum vorticity. The speed of light emerges as the sound speed in the vacuum condensate, with implications for variable-c cosmology. This constitutes the theoretical companion to "Harrison's Theorem of Anti-gravity" (DOI 10.5281/zenodo.18158165), which presents the experimental predictions and falsification criteria. Together, the papers establish a complete framework: theoretical derivation plus empirical test protocols.