Visible light communications (VLC) describes the use of the visible spectrum for optical communications and has the potential to provide vast new wireless spectrum free from license. Most existing work on VLC focuses on the optimization of the physical medium rather than the networking context in which VLC will be deployed. Future deployments of VLC are expected to coexist with current RF technologies; complicating performance analysis of these integrated, heterogeneous systems. In this paper, we propose an extension of the ns3 network simulator as a means to investigate the performance of these heterogeneous RF/VLC networks. The proposed VLC module realizes physical layer models characterizing the visible spectrum mapped to various optical modulation schemes. We also consider device mobility and orientation which uniquely impact an optical receiver. We describe the adopted physical models, the structure of the ns3 model implementation, and demonstrate performance assessment for an asymmetric RF/VLC scenario. In this case, the VLC downlink and the RF (WiFi) uplink are paired using the combination of our new ns3 VLC component and existing ns3 RF modules. Simulations demonstrate how this scenario can be studied in terms of VLC signal to noise ratio (SNR) and bit error rate (BER) parameters, and in the resulting network performance measured as goodput.