In this paper, we propose a video-based metropolitan positioning system (V-MPS) for providing centimeter-grade localization in vehicular networks. The V-MPS system integrates vehicular networks with installed roadside/intersection cameras to provide positioning information to vehicles on the road. V-MPS can estimate the precise position of a vehicle and detect the located lane for each vehicle. To the best of our knowledge, V-MPS is the first positioning system which provides the following features: 1) it achieves the centimeter-grade positioning accuracy for vehicular networks that can detect the located lanes of vehicles, 2) it employs existing roadside/intersection cameras that can keep additional construction cost low, and 3) it establishes an innovative infrastructure for applications of next-generation Intelligent Transportation System (ITS) that can make lane-level traffic control, collision avoidance, and vehicle navigation possible. With vehicular networks, the estimated positions can be broadcast to nearby vehicles. With roadside/intersection cameras, the lane positions of vehicles can be detected even if GPS is not accurate enough to provide lane-level localization. In addition, the traffic condition can be continually monitored to optimize driving efficiency for next-generation ITS. This paper demonstrates our current prototype.