The drillstring used in the oil and gas exploration is a complex structure due to the different forces acting on it. One of the primary sources of drillstring vibrations is the cutting forces caused by the drill bit contact with the rock formation. In some drilling applications, such as hole enlargement and underreaming, the source of the cutting action originates from the drill bit as well as the reamer which increases the dynamic complexity of the drillstring. This paper’s objective is to investigate the torsional vibration behaviors of the bottom hole assembly (BHA) under simultaneous drilling and reaming. More specifically, it addresses the effect of the reamer interaction with the wellbore during drilling operations on the overall torsional vibrations of the BHA. The BHA was modeled as a torsional shaft subjected to a localized external force due to the reamer cutting action, and a point load external force due to the drill bit interaction with the formation. The equation of motion was obtained using Hamilton’s principle, and modal expansion was used to solve the equation of motion. The results showed that the location of the reamer within the BHA plays an important role in vibrations response. It was found that vibration modes that exhibit symmetry within the reamer location show a negligible effect on the overall BHA torsional response. Reamers with aggressive cutters cause higher vibration response when compared with a drill bit with the same cutter aggressiveness. The simplified model reveals the significance of properly matching the drill bit and the reamer to reduce the overall BHA torsional vibrations.