This study investigates the impact of molecular disorder on vacuum Rabi splitting (VRS) in molecular polaritons. The research provides analytical expressions for linear absorption, transmission, and reflection spectra, along with a "sum" rule that enables the accurate extraction of collective light-matter coupling parameters from experimental data. The findings reveal that molecular disorder significantly influences VRS, with the splitting initially increasing, then saturating, and finally decreasing with increasing disorder. This behavior challenges the assumption that large VRS directly indicates ultrastrong coupling. For rectangular disorder, narrow sidebands appear alongside a broad central peak, indicating extended coherence lifetimes even in the presence of substantial disorder. The study also introduces a general sum rule applicable to all types of disorder, which is crucial for accurately determining collective light-matter coupling. The results highlight the importance of considering disorder effects when interpreting VRS in experimental data, as large VRS values may not necessarily imply ultrastrong coupling. The study provides a framework for understanding and extracting light-matter coupling parameters from experimental data, with implications for applications requiring prolonged coherence between cavity and molecules. The research underscores the complex interplay between molecular disorder and light-matter coupling, offering insights into the behavior of polaritons in disordered systems.This study investigates the impact of molecular disorder on vacuum Rabi splitting (VRS) in molecular polaritons. The research provides analytical expressions for linear absorption, transmission, and reflection spectra, along with a "sum" rule that enables the accurate extraction of collective light-matter coupling parameters from experimental data. The findings reveal that molecular disorder significantly influences VRS, with the splitting initially increasing, then saturating, and finally decreasing with increasing disorder. This behavior challenges the assumption that large VRS directly indicates ultrastrong coupling. For rectangular disorder, narrow sidebands appear alongside a broad central peak, indicating extended coherence lifetimes even in the presence of substantial disorder. The study also introduces a general sum rule applicable to all types of disorder, which is crucial for accurately determining collective light-matter coupling. The results highlight the importance of considering disorder effects when interpreting VRS in experimental data, as large VRS values may not necessarily imply ultrastrong coupling. The study provides a framework for understanding and extracting light-matter coupling parameters from experimental data, with implications for applications requiring prolonged coherence between cavity and molecules. The research underscores the complex interplay between molecular disorder and light-matter coupling, offering insights into the behavior of polaritons in disordered systems.