Despite the success to deliver increasingly large number of channels to millions of users, the current multi-channel P2P video streaming systems still suffer several fundamental performance problems, such as large start-up delays and poor performance for unpopular channels. To alleviate the impact of channel churn and resource imbalance, the View-Upload Decoupling (VUD)  P2P streaming design decouples peer downloading and uploading, and enables cross-channel resource sharing. However, VUD incurs upload bandwidth overhead and distribution swarm management cost. It is also challenging to adapt VUD distribution swarms in extreme peer churn scenarios, such as flash-crowd. In this paper, we propose ViVUD, a Virtual Server Cluster based VUD design. In ViVUD, a virtual server cluster consisting of bandwidth-rich peers is provisioned to improve the streaming quality of each channel. A virtual server cluster provides stable video feeds to boost peers newly joining a channel to reduce their start-up delays. To enable cross-channel bandwidth sharing, following the VUD design, virtual server clusters for unpopular channels are formed by bandwidth-rich peers watching popular channels. Through analysis and simulations, we show that, compared with the original VUD design, ViVUD incurs less upload bandwidth overhead, has lighter management requirement, achieves lower channel start-up delays, and adapts faster to flash crowds.