We aim to develop a microscopic theory to explain these observed VSC effects, especially focusing on understanding the resonance effect under normal incidence. Experimentally, only the resonance at normal incidence (𝑘 ‖ = 0) gives rise to VSC effects on the rate constant, while a red-detuned cavity that has a light-matter resonance at 𝑘 ‖ > 0 (oblique incidence) does not give any VSC effect. This observation strongly suggests that forming Rabi splitting is not a sufficient condition for achieving the VSC-modified rate effect. Despite recent theoretical progress , the resonant condition under normal incidence remains an unresolved question.