Microsoft has announced that the Zune software has been updated to be compatible with Vista:

Today (Tuesday the 19th of December) we’re issuing a 22 MB update to make Zune compatible with Windows Vista. The update also improves the Zune software installation process, addressing the issues that some of the known issues users reported. Like Jason R said recently “It is plumbing stuff, but it is stuff customers will notice and appreciate.” So you should install the update, even if you’re not running Vista.

You can install it via Microsoft auto-update, or go to Zune.net to get it.