With urbanization, anthropogenic water vapor emissions have become a significant component of the urban atmosphere. Fossil fuel combustion-derived vapor (CDV) is a primary source of these emissions. Owing to the notably low CDV d-excess, stable hydrogen and oxygen isotopes are promising for distinguishing CDV from natural sources. Considering the limitations of in situ observations, this study aims to explore the feasibility of using IsoRSM, an isotopically enabled regional atmospheric model, to simulate CDV emissions in urban areas in winter. Two experiments were conducted: one in Salt Lake City in January 2017 and another in Beijing in January 2007. The simulation results showed that the CDV addition significantly reduced the water vapor d-excess, particularly when the boundary layer was stable. The simulation with CDV emissions aligned better with the time series of in situ observations in Salt Lake City. The modification led to a more pronounced positive correlation between vapor d-excess and specific humidity, which was similar to the observation of Salt Lake City. The CDV inclusion significantly increased the vapor d-excess variability with varying wind directions in both sites. However, in Beijing, the underestimation of d-excess variation from natural sources caused a bigger discrepancy between the observed and simulated d-excess and CDV fraction. Thus, though there were still biases, the inclusion of CDV could improve the accuracy of isotopic simulation in the urban regions where CDV was one of the controlling factors of vapor d-excess.