I've checked the official site.
http://aisp.jp/home.html
Now the game is open to anyone absolutely for free.
Mister Bushido
As far as I know, windows vista contains both direct x 9 and 10. Or rather, the latest version of direct x already contains the previous ones. So, basically, you don't have to downgrade anything.
You might want to switch to japanese settings just in case. Though I'm not sure if it can solve the directx problem.
http://www.microsoft.com/japan/windo...x/default.mspx
Direct x can be downloaded from MS site. However, I should say that I personally didn't install anything, the game just works from the start.
I personally use my Sony laptop with vista and the game works perfectly with this.
Meanwhile, today I faced considerable slow downs in the game. Apparently, it is due to high number of visitors. For example, you can't normally watch tv - the video doesn't show up.