Virtual content in Augmented Reality (AR) applications can be constructed according to the designer's requirements, but real environments, are difficult to be accurate control or completely reproduce. This makes it difficult to prototype AR applications for certain real environments. One way to address this issue is to use Virtual Reality (VR) to simulate an AR system, enabling the design of controlled experiments and conducting usability evaluations. However, the effectiveness of using VR to simulate AR has not been well studied. In this paper, we report on a user study (N=20) conducted to investigate the impact of using an VR simulation of AR on participants' task performance and cognitive workload (CWL). Participants performed several office tasks in an AR scene with virtual monitors and then again in the VR-simulated AR scene. While using the interfaces CWL was measured with Electroencephalography (EEG) data and a subjective questionnaire. Results showed that frequent visual checks on the keyboard resulted in decreased task performance and increased cognitive workload. This study found that using AR centered on virtual monitor can be effectively simulated using VR. However, there is more research that can be done, so we also report on the study limitations and directions for future work.