Using 2D IR to resolve ultrafast chemical dynamics with specific initial states, we quantified the energy exchange dynamics in Fe(CO)5 under VSC. We showed that when UP is excited under VSC, IVR is promoted and pseudorotation is suppressed compared to the bare molecular system. However, pumping the dark reservoir modes under VSC led to little change in the dynamics compared to outside the cavity. Because the population at thermal equilibrium resides predominantly in the dark modes, the overall influence of VSC on Fe(CO)5 should be negligible without external (e.g., laser) pumping. Yet, the present results show an important insight to unify the works reporting VSC-modified reactions and the ones reporting or predicting the oppositeregardless of how reactions behave under thermally-activated conditions, the basic concept of VSC-modified chemistry works: populated polaritons can influence chemical dynamics. These findings suggest that the future of VSC-modified thermal chemistry lies in controlling the dark modes, with either reducing the number of dark reservoir modes, e.g., through cavity miniaturization, or making dark modes more delocalized through heterogeneity .