A highly sensitive multimode-interference-based refractive index sensor is reported here by shining an optical vortex beam. The sensor probe is formed by splicing a length of no-core fiber in between two air-core vortex fibers. The coupling characteristics of various modes inside the sensor and their effect on sensing properties are numerically analyzed. Simulation results show that the sensing scheme proffers a maximum sensing resolution of 7.59 × 10−6 and 4.18 × 10−6 RIU for no-core fiber length of 29.40 and 44.60 mm, respectively. Because of its high sensitivity, the study has potential applications in the chemical and biological sensing fields.