This study investigates the impact of molecular disorder on vacuum Rabi splitting (VRS) in molecular polaritons, a fundamental measure of collective light–matter coupling. The research provides precise analytical expressions for linear absorption, transmission, and reflection spectra, along with a "sum" rule, which offers a straightforward method to extract accurate collective light–matter coupling values from experimental data. Key findings include: 1. **VRS Trends with Disorder**: VRS initially increases with disorder, reaches a saturation point, and then decreases to zero for a wide range of disorder. This behavior is consistent across different disorder distributions (Lorentzian, Gaussian, and rectangular). 2. **Sum Rule**: A generalized sum rule is introduced, which can be used to determine the collective light–matter coupling when both absorption and transmission spectra are accessible. This rule is applicable to any form and strength of disorder. 3. **Rectangular Disorder**: For rectangular disorder, the study observes the emergence of narrow sidebands alongside a broad central peak, indicating extended coherence lifetime even in the presence of significant disorder. This phenomenon suggests potential applications in systems requiring long-lived coherences between the cavity and molecules. 4. **Model and Analysis**: The study uses a Tavis–Cummings model to derive analytical expressions for the linear response of molecular polaritons under various disorder distributions. Numerical simulations are used to validate the analytical results, showing strong agreement. 5. **Conclusion**: The research enhances our understanding of VRS in disordered molecular systems and provides a reliable framework for extracting light–matter coupling parameters with high accuracy from experimental data. The findings also highlight the importance of considering the sum rule in interpreting VRS data, especially in the context of ultrastrong coupling regimes. Keywords: molecular polaritons; strong light-matter coupling; disorderThis study investigates the impact of molecular disorder on vacuum Rabi splitting (VRS) in molecular polaritons, a fundamental measure of collective light–matter coupling. The research provides precise analytical expressions for linear absorption, transmission, and reflection spectra, along with a "sum" rule, which offers a straightforward method to extract accurate collective light–matter coupling values from experimental data. Key findings include: 1. **VRS Trends with Disorder**: VRS initially increases with disorder, reaches a saturation point, and then decreases to zero for a wide range of disorder. This behavior is consistent across different disorder distributions (Lorentzian, Gaussian, and rectangular). 2. **Sum Rule**: A generalized sum rule is introduced, which can be used to determine the collective light–matter coupling when both absorption and transmission spectra are accessible. This rule is applicable to any form and strength of disorder. 3. **Rectangular Disorder**: For rectangular disorder, the study observes the emergence of narrow sidebands alongside a broad central peak, indicating extended coherence lifetime even in the presence of significant disorder. This phenomenon suggests potential applications in systems requiring long-lived coherences between the cavity and molecules. 4. **Model and Analysis**: The study uses a Tavis–Cummings model to derive analytical expressions for the linear response of molecular polaritons under various disorder distributions. Numerical simulations are used to validate the analytical results, showing strong agreement. 5. **Conclusion**: The research enhances our understanding of VRS in disordered molecular systems and provides a reliable framework for extracting light–matter coupling parameters with high accuracy from experimental data. The findings also highlight the importance of considering the sum rule in interpreting VRS data, especially in the context of ultrastrong coupling regimes. Keywords: molecular polaritons; strong light-matter coupling; disorder