Even though most consumer based computers come with Vista installed, I think you will find that most corporations are ordering machines with XP Pro. For example if you go to Dell or HP and look at the Business Class workstations, desktops, and laptops you will see that they still come with XP (Even though you can get Vista as an option). This resistance by corporations is why you are seeing the major CAD suppliers not in any big rush to add Vista support. The same thing happened when XP was released. Here is a couple of good articles about why corporations are waiting.
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With all of this being said, you can always trying installing your tool on Vista and see what happens. If you do this, I wouldn't expect much supports from the Vendors if they are not officially supporting it.