To explore the effectiveness of network protocols and services, it is desirable to deploy and evaluate them under a realistic network setting. However, experimentation in production networks is difficult in practice, since it may involve modifications of underlying routers or other hardware components and disrupt the normal operations of existing protocols. We propose VegaNet, a virtual network architecture that provides an experimental platform atop a physical production network. VegaNet uses a lightweight probing mechanism to provide a consistent connectivity view as in the underlying physical production network so that experimentation can be performed under realistic network conditions. It uses virtualization to host multiple experiments on a single physical machine, while reflecting the current connectivity status to each hosted experiment in an accurate and timely manner. We prototype VegaNet and empirically evaluate its effectiveness atop a real-life production network that is currently deployed nationally.