This study examines the effect of mass ratio $\left(m^{*}\right)$ on vortex-induced vibrations of circular cylinders initially in tandem arrangement undergoing rigid collision. A finite volume approach with a diffuse interface immersed boundary method analyzes wake dynamics at an initial gap ratio $g/d=1.5$, over the reduced velocity range $3\le U^{*}\le 15$, and Reynolds number $\text{Re}=200$. The structural response reveals a monotonically increasing amplitude $\left(A_y^{*}\right)$ with $U^{*}$, indicative of lock-in within the synchronization regime before saturating beyond $U^{*}\approx 10$, while a systematic decrease in $A_y^{*}$ with rising $m^{*}$ highlights the greater inertial resistance of heavier structures. Results show a transition from chaotic wakes at $m^{*}=1$ to organized vortex patterns at higher $m^{*}$. At low $m^{*}$, collisions induce irregular shedding and nonlinear oscillations. As $m^{*}$ increases, wake patterns stabilize into periodic modes, leading to organized displacement. The upstream cylinder undergoes strong sign reversals in the time-average lift coefficient, while the downstream cylinder largely assumes the opposite sign due to gap-flow deflection and shielding, with matching signs occurring only at pinned, near side-by-side collision states where the wake momentarily reorganizes into a symmetric dipole. Across all $m^{*}$, the circulation $\left({\mathrm{\Gamma }}_z\right)$ strength increases with $U^{*}$ and peaks near $U^{*}\approx 6$, then declines, with heavier cylinders consistently yielding higher ${\mathrm{\Gamma }}_z$. Phase-space analysis shows irregular interactions at low $m^{*}$, stabilizing into consistent antiphase synchrony as $m^{*}$ increases.

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• Vortex dynamics
• Immersed boundary methods