This messenger was covered on this blog around one year back and some 6 months back they added some new features to it.
As of today, 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.
This decision will help us increase efficiencies on our team and deliver one consistent, full-featured product for all of our 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:
http://messenger.yahoo.com/download/win
Yahoo! Messenger for Vista was never a full featured application, so I assume most of the people were using both of them (including me). But now, no more confusion. Just use Yahoo! Messenger 9.0.
One thing I wonder though,
“Does slow economy has to do anything in the decision of discontinuing this Vista Messenger?”
But hey, I don’t even remember using Yahoo! messenger for a long time now because on Vista I use the new Windows Live Messenger Beta and 95% time I use Ubuntu, so I use Pidgin.
Which IM application you use:

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Ah..I was so excited back then about this one.. luckily I rarely log on to my yahoo account anymore
[...] Yahoo Messenger for Vista is killed [...]