Vibrational strong coupling (VSC) gives rise to delocalized superpositions of molecular vibrations and electromagnetic modes (cavity modes), known as molecular vibrational polaritons . Recently, VSC has arisen as a promising handle to manipulate chemical reactions in condensed phases . Extensive experimental evidence has shown that, without photoexcitation, reaction rates can be either accelerated or decelerated by VSC, and reaction selectivity can even be altered . While much effort has been devoted to providing a sound explanation for VSC-modified chemistry, consensus between theory and experiments is still missing . Though it is clear that polaritons are different from bare molecular states, and thereby have the potential to modify chemistry, dark modes, which greatly outnumber polaritons, have the same excitation energies as the uncoupled vibrational modes . Hence, some theoretical work predicts that reactivity in the collective VSC regime is similar to that outside an optical cavity , which is consistent with a couple of recent experimental results showing no modification of reactions from VSC . Resolution of the discrepancies is hindered by the following factors: most reactions studied so far are quite complex, i.e., involve multiple steps or are diffusion limited, and reactions involving both dark modes and polaritons are probed together . To delineate the effect of VSC, it is therefore critical to study elementary reactions and use a technique that can differentiate the contributions from polaritons and dark modes.