Vista Ultimate & DX9

I installed Call of Duty World at War which comes with DX9. I thought the game would automatically detect it is Vista then wont ask for a DX9 install and settle for Vista's DX10.

But during installation, it still asked for DX9 install, so I said yes and it installed. After that's done, I issue DXDIAG from Start>Run, fortunately, Vista still keeps DX10.

So would the game now run on DX 10?

It should work, I've ran old DX7 and DX8 games on Vista.

Updates for DirectX9 still apply to vista, they don't overwrite any other version of it. They release an update every few months that ends up getting bundled with games as most people don't update manually.