The complex vortex-induced vibrations (VIV) dynamics of tandem diamond cylinders of equal mass ratio (= 10) at Reynolds number (= 100) and reduced velocity between 1 and 12 using a two-dimensional, stabilized space-time finiteelement computation is investigated. The study unveils the profound influence of vortex-shedding on various aspects of VIV, including vibration amplitude and vortex-shedding pattern. Three distinct vortex modes were identified in the gap region between the cylinders, each influencing the oscillation amplitude of the downstream cylinder differently. The oscillation amplitude of the downstream cylinder varied with the upstream cylinder's vortex shedding dynamics: merging vortices in the gap region reduced the amplitude, while increased upstream oscillations, leading to more shed vortices, enhanced it. Further downstream, vortex coalescence in the far wake was observed only at select reduced velocity values, and an intriguing 'wake deflection' was noted at certain reduced velocity values.