This paper investigates the dynamic characteristics and vibration transmission behaviour of harmonically forced oscillators coupled via an inerter-based joint. The inerter is a recently proposed passive mechanical element that can be used for vibration suppression purpose. The coupled oscillators represent simplified models for the dominant modes of engineering structures such as beams and plates, while the inerter-based joint serves as the vibration transmission path between the two subsystems. The effects of the inerter on the level of vibration transmission are examined for enhanced design of suppression devices. The steady-state dynamic responses of the oscillators and vibration power flow between them are derived analytically. Vibration transmission between the two subsystems is evaluated by both the force transmissibility and time-averaged vibration power flow transmission. It is shown that the use of the inerter in the joint can reduce force and vibration power flow transmission over a large band of excitation frequencies. It is found that the addition of the inerter can also introduce anti-resonances in the frequency-response curves and in the curves of the force transmissibility, where the vibration transmission can be suppressed at specific excitation frequencies. It is shown that force transmissibility reduces at low frequencies and tends to an asymptotic value as the excitation frequency decreases. These findings provide a better understanding of the effects of inerters in the joint on vibration transmission and benefit future designs of mechanical joints of coupled structures for vibration mitigation purpose.