I strongly doubt it. DX integrates very tightly with Windows. Removing DX from WinXP is a nightmare. Downgrading DX10 and forcing Vista to use a version of DirectX that is not even designed to integrate with it? I don't think its possible considering how hardass Vista is with low-level drivers.
Addenum
From an official dev:
QUOTE > Yes, you will. You still have DX10 installed, but you also have the
> components from 9.0c that the games need.
More correctly: Windows Vista includes a DirectX runtime that supports
Direct3D 9.0c, Direct3D9Ex, and Direct3D 10, as well as emulating older
versions of Direct3D. The game's copy of D3DX9 is needed, but everything
else is already there.
Read "Graphics APIs in Windows Vista"
http://msdn2.microsoft.com/en-us/library/bb173477.aspx and check out the
latest DirectX FAQ
http://msdn2.microsoft.com/en-us/library/bb219721.aspx
Q: Will DirectX 10 be available for Windows XP?
A: No. Windows Vista, which has DirectX 10, includes an updated DirectX
runtime based on the runtime in Windows XP SP2 (DirectX 9.0c) with changes
to work with the new Windows Display Driver Model (WDDM) and the new audio
driver stack, and with other updates in the operating system. In addition to
Direct3D 9, Windows Vista supports two new interfaces when the correct video
hardware and drivers are present: Direct3D9Ex and Direct3D10.
Since these new interfaces rely on the WDDM technology, they will never be
available on earlier versions of Windows. All the other changes made to
DirectX technologies for Windows Vista are also specific to the new version
of Windows. The name DirectX 10 is misleading in that many technologies
shipping in the DirectX SDK (XACT, XINPUT, D3DX) are not encompassed by this
version number. So, referring to the version number of the DirectX runtime
as a whole has lost much of its meaning, even for 9.0c.
The DirectX
Diagnostic Tool (DXdiag.exe) on Windows Vista does report DirectX 10, but
this really only refers to Direct3D 10.
Q: What changes were made to the DirectX runtime for Windows Vista?
A: The primary changes were to support the new Windows Display Driver Model.
For details on the new driver model, impacts on Direct3D 9, and on the two
new graphics interfaces, Direct3D 9Ex and Direct3D 10, review Graphics APIs
in Windows Vista.
DirectSound was updated to expose the capabilities of the new Windows Vista
audio driver stack, which supports multi-channel software buffers. The
legacy Direct3D Retained Mode API was completely removed from Windows Vista.
DirectPlay Voice was also removed, as well as
DirectPlay's NAT Helper.
--
Chuck Walbourn
SDE, Game Technology Group[/quote]
See bolded parts. It seems a lot of stuff has been been removed from the DX9.0c version shipped with Vista. Removed NAT Helper? Oh my. How does Alleg work at all then?
This post refers to Vista x64. Also, from this post, it can be understood that DX10 = DX9.0c + D3D10 - some stuff.
.. )