We present a theory to explain the current VSC experiments, focusing on the origin of the resonance condition at normal incidence. The theory clearly explains the resonance condition for the observed VSC effect and provides an explanation of why the resonant effect occurs only at the normal incident angle. In particular, we find that the cavity-modified rate constant 𝑘 VSC can be expressed as the coupling strength multiplied by the accumulated spectral function 𝒜(𝜔) of the cavity, where 𝒜(𝜔) peaks at 𝜔 c (when 𝑘 ‖ = 0, i.e., bottom of the dispersion band). For a 1D FP cavity, this is caused by a van-Hove-type singularity (Eq. 22) in the DOS of the photonic modes. For a 2D FP cavity, we found that one needs to additionally consider the photon loss associated with 𝑘 ‖ direction (Eq. 26), which creates the peak of 𝒜(𝜔) at 𝜔 c . As such, the oblique incidence still has the spectral function peaked at the 𝜔 c , not at the higher incident angle. This theory explains why Rabi splitting is not a sufficient condition to achieve a VSC modified rate effect, providing a new insight into the mechanistic understanding of VSC modification.