Ride comfort improvement and motion sickness reduction are gaining attention given recent technological trends, such as the advancement of automated driving systems, the introduction of in-vehicle digital devices, and the daily use of mobile devices in vehicles. As a countermeasure, mathematical models predicting motion sickness were proposed. Among them, models based on sensory conflict or subjective vertical conflict theories were developed. These models can successfully describe the tendency of motion sickness in several scenarios involving various vestibular inputs or head movements, including carsickness. Almost all models are based on human motion perception with an internal model hypothesis. It has advantages when expanded to model sickness caused by multi-sensory inputs. Some expansions of the models to include the effects of visual information and motion prediction on motion sickness have been made. However, the motion perception calculated by the models has not been investigated, while the motion sickness output has been evaluated for various scenarios, including carsickness. Therefore, as the first step, we focused on vestibular motion sickness. Herein, 6 DoF SVC models of vestibular motion sickness were investigated to determine whether they could describe motion sickness incidence (MSI) as well as motion perception reported in the literature through parameter optimization. In this investigation, four different structures of 6 DoF SVC models, with or without integrals in the feedback process of the observer-theoretic structure, were explored, including the original 6 DoF SVC model. We found that multiple models with optimized parameters could describe the tendency of MSI as well as motion perception.