We present a hybrid Visible-Light Communication (VLC) and Radio-Frequency (RF) communication system based on an open-source platform, which is integrated at the Medium-Access-Control (MAC) layer. Downlink data transmissions of the system use VLC with a commercially available lighting source, and uplink transmissions use RF. An auxiliary, low-power infra-red (IR) control channel is also implemented on the uplink to transmit acknowledgement signals for VLC data packets. We experimentally demonstrate that the system has low latency, and can achieve end-to-end TCP/IP throughput comparable to single antenna, 20 MHz 802.11 links when operated in isolation, and provide a 70% increase in total throughput when operating concurrently and in the same channel as a legacy 802.11 TCP/IP link. We also show that the RF signals of our system can achieve fair usage of the RF spectrum with an independent 802.11 network with UDP transmissions. In addition to addressing some practical challenges of implementing hybrid, asymmetric VLC-RF systems, our system is open-source, and requires no custom components and can serve as a reference for researchers who wish to construct and analyze hybrid VLC-RF systems.