This paper numerically investigates the influence of a fixed downstream control cylinder on the flow-induced vibration of an elastically-supported primary cylinder. These two cylinders are situated in a tandem arrangement with small dimensionless centre-to-centre spacing ($L/D$, $L$ is the intermediate spacing, and $D$ is the cylinder diameter). The present two-dimensional (2D) simulations are carried out in the low Reynolds number ($Re$) regime. The primary focus of this study is to reveal the underlying flow physics behind the transition from vortex-induced vibration to galloping in the response of the primary cylinder due to the presence of another fixed downstream cylinder. Two distinct flow field regimes, namely steady flow and alternate attachment regimes, are observed for different $L/D$ and Re values. Depending on the evolution of the near-field flow structures, four different wake patterns - `2S', `2P', `2C', and `aperiodic' - are observed. The corresponding vibration response of the upstream cylinder is characterized as interference galloping and extended vortex-induced vibration. As the $L/D$ ratio increases, the lift enhancement due to flow-induced vibration is seen to be weakened. The detailed correlation between the force generation and the near-wake interactions is investigated. The present findings will augment the understanding of vibration reduction or flow-induced energy harvesting of tandem cylindrical structures.
