Video streaming has become a popular form of transferring video over the Internet. With the emergence of mobile computing needs, a successful video streaming solution demands 1) uninterrupted services even with the presence of mobility and 2) adaptive video delivery according to current link properties. In this paper we study the need and evaluate the performance of adaptive video streaming in vertical handoff scenarios. We use Universal Seamless Handoff Architecture (USHA) to create a seamless handoff environment, and use the Video Transfer Protocol (VTP) to adapt video streaming rates according to "Eligible Rate Estimates". Using testbed measurements experiments, we verify the importance of service adaptation, as well as show the improvement of user-perceived video quality, via adapting video streaming in the vertical handoffs.