Sorry to tell you but microsoft products cannot be rolled back to an older version wile keeping the newer one. Its the same thing like installing Win95 on WinXP. But one way can be to delete the system file for windows media player from startup command promot using a floppy then going for media player 9 might solve this issue.
WMP 11 on Vista is not to my liking but since it's embedded in the OS it can't be deleted. Is there a way to install WMP 9 somehow and fool Vista into thinking it's a later version?