The standard astrophysical model attributes the stability of Main Sequence stars entirely to the hydrostatic equilibrium between gravitational collapse and outward thermal/radiation pressure. However, this purely kinematic and thermodynamic balance lacks a fundamental negative-feedback mechanism governing the precise temporal rate of the fusion process itself. In the Primary Energy (PE) framework, the physical vacuum is modeled as a viscous superfluid medium whose local density and kinematic viscosity are strictly determined by the localized energy density. We propose a novel mechanism for stellar stability: the "Vacuum Thermostat". As nuclear fusion releases immense thermal energy within the stellar core, the local vacuum energy density (rho_E) drastically increases. This spike in density induces a corresponding increase in vacuum viscosity, which physically dampens the frequency of internal atomic processes, phenomenologically observed as extreme time dilation. This induced time dilation acts as a direct throttle on the nuclear reaction rate. We derive the mathematical formulation of this hydrodynamic feedback loop, demonstrating how it ensures the billions of years of stable hydrogen burning in stars like the Sun, and explaining the rapid burnout of hypermassive stars where extreme physical pressure overcomes the viscous threshold. Quantitative estimates for the Sun and massive stars are provided, and observational tests via helioseismology and solar neutrino flux are discussed.