Cooling demand dominates the energy consumption of buildings in hot-summer/warm-winter zones. Vertical green façades (VGFs) are an effective design strategy to reduce the cooling load in these regions. Many studies have investigated the cooling benefits of VGFs; however, the lack of accurate and low-cost co-simulation methods limits their application as it is difficult for architects and engineers to use them to evaluate the energy consumption of buildings. To address this issue, we developed a dynamic heat transfer (DHT) model for VGFs based on the Beer–Lambert Law and Penman–Monteith Equation. We then proposed a co-simulation approach for VGFs with EnergyPlus by using an additional heat source term and energy management system. We subsequently conducted field measurements to validate the accuracy of the DHT model and the reliability of the co-simulation approach. Finally, we created simulation scenarios in four typical cities in the hot-summer/warm-winter zone of China and analysed the effects of a VGF on the cooling load of an office building. The results indicated that the VGF reduced the room cooling load by 11.7–18.4%. Furthermore, improving the wall insulation only led to a 0.3–8.4% reduction in the room cooling load. Moreover, the cooling load saving rates of rooms with a VGF and well-insulated walls were 6.6–15.8%. Our research is likely to prove useful to future researchers investigating VGFs using DHTs and building designers attempting to effectively incorporate VGFs into architectural plans.