This comprehensive work, structured into thirteen sections, delves into the Variable Speed of Light (VSL) hypothesis, offering an innovative framework for understanding cosmological phenomena. The introduction and prologue set the stage by addressing the motivations behind VSL and the misinterpretations of Einstein's second postulate. While Einstein posited that the speed of light in a vacuum, 'c,' must be determined experimentally, this has been misinterpreted to mean that 'c' is universally constant under all conditions. Instead, this work argues that 'c' might locally vary based on the properties of each specific vacuum, challenging the notion of a true-vacuum and suggesting it behaves more like a low-density medium rather than completely empty space.Section 1 explores the relationship between permittivity, permeability, and VSL, laying the groundwork for understanding how changes in vacuum properties could affect light speed. Section 2 examines the vacuum itself, proposing that its characteristics can influence 'c' and thus impact our understanding of space and time. Section 3 extends this by discussing different types of media within the vacuum and their potential effects on light speed.Section 4 analyzes the implications of considering the speed of light as locally constant rather than universally fixed, which could lead to significant revisions in both Special and General Relativity. Section 5 compares VSL with the traditional inflationary model, suggesting that VSL could provide alternative explanations for the early universe's rapid expansion. Section 6 addresses the Hubble Tension, exploring how VSL could reconcile discrepancies in measurements of the Hubble constant.Sections 7 and 8 discuss the practicalities of implementing VSL and propose experiments and observations to test its predictions. Section 9 focuses on empirical verification, emphasizing the need for robust observational data to validate the VSL hypothesis. Section 10 presents a mathematical model for VSL, highlighting its potential to reshape our understanding of the universe's expansion.The final conclusions summarize the key findings, stressing the importance of further investigation and experimental verification. This work aims to challenge established concepts, suggesting that VSL could provide new insights into cosmology and urging the scientific community to explore this hypothesis more deeply.