The only noticeable difference between Windows Vista and Windows 7 was the elimination of Windows Vista's constant consumption of memory / bad memory management.
It's been awhile, but IIRC they were virtually identical UI's. Windows 7 just "fixed" Vista's memory & performance issues.
For Windows 9 to be to Windows 8 what Vista was to Windows 7, that means the UI stays the same and the performance issues are "fixed."
Granted Windows 8's conflicting UI's (Metro vs. Desktop) sucks rocks, but the performance over Windows 7 is really quite good.
So I think your analogy is wrong. Windows 9 will be to Windows 8 what Windows XP was to Windows 3.11/Windows for Workgroups.
Was just speaking in a general, overall/high level way. Wasn’t speaking of any specific UI or performance differences.