Human presence plays an important role in the operation of HVAC systems. Traditional HVAC systems usually rely on fixed occupancy schedules, leading to a substantial amount of wasted cooling energy, particularly in tropical climate countries such as Vietnam. To evaluate the impact of occupant presence under realistic conditions, this study conducts a simulation-to-simulation (Sim-vs-Sim) case study of a typical office in Hanoi, Vietnam, using CO₂-derived occupancy schedules extracted from a two-week field deployment. Unlike a single representative day, the proposed framework is evaluated across multiple days spanning both mild and high cooling-demand conditions. Results show that the energy-saving effect of occupancy-driven scheduling is conditional on ambient cooling demand: on days with negligible cooling load, static and dynamic schedules produce identical energy use, while on the highest cooling-demand day observed, the dynamic occupancy schedule reduced total daily energy consumption by 7.51% and peak cooling load by 13.95% compared to the static baseline. These findings suggest that occupancy-aware HVAC control is most impactful during high cooling-demand periods, and that its benefit should be reported and interpreted conditionally rather than as a single aggregate figure.