Not even a wormable flaw could convince them to patch Vista, apparently (assuming it's not somehow magically invulnerable when the versions before and after it weren't).
Maybe the market share of Vista is so small, that Microsoft doesn't bother releasing a patch for it. There's no patch for Windows 8, either.
As someone commented above, there's a footnote about how Vista users can use the Server 2008 patch, I was more amused that in an apparently-complete enumeration of versions in a table, they just...left it off.