Rapid urban expansion can significantly alter urban microclimate. In recent years, the phenomenon of urban dry islands (UDIs) has attracted increasing attention, but there remains a lack of understanding regarding the impact of urbanization on UDIs in different urban agglomerations (UAs). In this study, vapor pressure deficit (VPD) was used as the humidity indicator, and meteorological data from 20 UAs were used to explore how atmospheric temperature and humidity control urban dryness variability in China. Our analysis revealed that VPD trends are increasing with a median rate of 0.121 hPa/decade and 0.103 hPa/decade for urban and rural areas, respectively. The urbanization effect is more significant in coastal UAs. Furthermore, the difference in VPD between urban and rural areas obviously varied on a seasonal scale, with the largest difference observed in summer (i.e., −0.63 hPa on average and up to 2.66 hPa). Notably, our findings suggest that changes in local atmospheric water content primarily control the response of VPD to urbanization. In summer, the average contribution of temperature is 0.118 hPa, while the average contribution of relative humidity is 0.723 hPa. Overall, these results have important implications for our understanding of the impacts of urbanization on local climate.