According to Yahoo Messenger blog – Yahoo! Messenger for Vista will no longer be available for download from the Yahoo! Messenger website. We have discontinued stand-alone releases of the Yahoo! Messenger for Vista application in order to focus on delivering one Windows experience that is optimized for all Windows users.
If you’re using Yahoo! Messenger for Vista, you can continue to use it however we will not add any more features or provide product support. To enjoy full Yahoo! Messenger functionality including photo sharing, webcam and chat rooms, we recommend downloading and installing the latest version of Yahoo! Messenger 9.0, which also works on Windows Vista